DJI Agras T50 OEM DB1560 Battery Motherboard Fuse

Original DB1560 Battery Motherboard Fuse (400A, 70V) | High-Current Battery Protection Fuse | DJI Agras T50 & T40 Intelligent Battery Repair Component | Battery Electronics Safety Hardware | Original Replacement Fuse

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🇺🇸 U.S.A. FIRST

Ares Acres LLC is a U.S.-based authorized DJI Agras reseller supporting American agricultural drone operators with genuine DJI Agras original replacement parts, accessories, battery components, and field-ready inventory.

FREE & FAST Shipping is included for all U.S. customers.

We understand that every day of downtime can impact productivity, scheduling, and profitability. That's why we focus on providing fast fulfillment, responsive customer support, and reliable access to the parts operators need most.

A blown fuse is the cheapest failure a DB1560 battery can have — but until it's replaced, a multi-thousand-dollar intelligent battery sits dead on the rack instead of flying in the rotation.

That is why Ares Acres focuses on fast fulfillment, responsive support, reliable sourcing, and genuine DJI components that help American drone operators keep their aircraft and battery systems working when it matters most.

For commercial applicators, custom spraying businesses, growers, battery technicians, and fleet operators, this small component is the difference between a quick bench repair and a lost battery.

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✅ DJI Agras T50 / T40 Compatibility Guarantee

• 🇺🇸 USA Compatibility Guarantee
• 👍🏻 Satisfaction Guarantee
• 💵 Money Back Guarantee
• 📦 Same Day Shipping & Tracking

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📦 Package Includes

• 1 × DJI DB1560 Intelligent Battery Motherboard Fuse (400A / 70V)
• Pre-installed mounting contacts for easy replacement
• Anti-static packaging for safe handling
• Basic installation guidelines
• Direct-fit original replacement component

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🔧 Component Overview

The DJI Agras DB1560 Battery Motherboard Fuse is the original high-current protection component used in DJI Agras T50 and T40 DB1560 intelligent flight batteries. Rated at 400A continuous and 70V, this precision fuse sits between the battery's enormous stored energy and the sensitive control electronics that manage it.

When an overload, short circuit, or electrical fault occurs, the fuse sacrifices itself in milliseconds — interrupting the circuit before the fault can destroy the motherboard, damage the cells, or create a thermal event. It is the battery's last line of electrical defense, and it is designed to blow so nothing else does.

Batteries in commercial agricultural service face conditions that put this protection to work, including:

• Repeated high-power discharge under full payload
• Rapid charging cycles between missions
• High-current spikes during aggressive flight loads
• Transportation vibration and handling stress
• Moisture, dust, and chemical residue exposure
• Heat cycling through charge and discharge
• High-hour commercial deployment

Built to DJI OEM specifications, this fuse restores the exact protection rating the DB1560 battery system was engineered around — not an approximation from a generic parts bin.

Test the fuse across the fuse, not across the pack. A fuse is a two-terminal device that is either continuous or it is open, so the measurement is simple, but only if you put your probes on the fuse terminals themselves. Testing at the pack's output connector puts the board, the switching hardware and every protection circuit in series with your reading, and any one of those will give you an open circuit for reasons that have nothing to do with the fuse. Continuity terminal to terminal on the fuse body means the element is intact and the fault is somewhere else entirely.

Read the blown fuse before it goes in the bin, because its appearance is the best evidence you will get about what happened. An element that has parted in one place with little discoloration and an otherwise clean body usually opened under a sustained overload, which points you at the load, at a thermal problem, or at something drawing more than it should over a period of time. A body that is deeply discolored, blackened, spattered inside, bulged or cracked took a hard short circuit, and that points at a physical fault such as a damaged busbar, a chafed or pinched cable, or water tracking across a live surface.

Look at the fuse contacts and the cable next to them, because a fuse can open at a current it should have carried comfortably. A high-resistance joint at a fuse terminal generates heat every time the pack works, that heat goes into the fuse element, and an element that is already running hot opens sooner than its rating suggests. The signs are specific: discoloration or oxide bloom on the contact faces, a dulled or stiffened section of cable insulation right at the terminal, and sometimes a faint smell of hot insulation. If you find that, the joint is the fault and the fuse was the casualty.

Separate the three failures that all look identical at the button. A pack that shows nothing when you press it can be a blown fuse, a control board that will not wake, or cells that have gone too low to start. A qualified technician can sort them quickly: with the pack isolated and opened per procedure, a healthy cell stack behind an open fuse is a bench repair, a continuous fuse with a healthy stack and still no recognition puts the fault on the electronics, and a stack that is low or badly out of balance is a cell problem and usually the end of the pack.

Find the second fault before you fit the new fuse, because the fuse is the report and not the fire. Go over the busbars and every cable lug for looseness, discoloration or deformation, follow every cable where it passes an edge, a clamp or a fold and look for a compressed or cut jacket, check the output connector contacts for pitting or melted plastic, and look across the board around the fuse position for heat discoloration, lifted solder or corrosion tracking. Fitting a fuse into an undiagnosed pack usually buys you one more failure at a worse moment.

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⚙️ Functional Purpose

The motherboard fuse protects the DB1560 battery's power control electronics by interrupting the circuit instantly when current or fault conditions exceed safe limits.

This component helps:

• Protect the battery motherboard from overload damage
• Interrupt short circuits before they propagate
• Guard against electrical fault conditions
• Support safe high-current discharge under load
• Support safe rapid-charging operation
• Preserve the battery management system electronics
• Reduce the risk of thermal events
• Restore blown-fuse batteries to service
• Return valuable batteries to the fleet rotation
• Protect the larger investment behind a small component
• Support professional battery repair standards
• Maintain reliable field performance

A fuse that blew did its job.

Replacing it with the correct original rating means the protection is there the next time it's needed — an over-rated or bypassed fuse turns the next fault into a destroyed battery.

Understand what you cannot switch off before the pack is opened. A DB1560 has no off position on the inside: the cell stack is live at all times and it can deliver enormous current into a short with no warning and no time to react. Work on a non-conductive bench, remove watches, rings and any metal jewelry, use insulated tools, keep one hand out of the work where you can, and have your fire response ready before you start rather than after. This is trained technician work under high-energy battery procedures, and none of it is optional.

Never lay a tool down across the fuse position. The fuse pads are the one place in the pack where a bridging piece of metal puts a dead short directly across the full cell stack with no protection left in the circuit, and that is exactly the condition the fuse exists to prevent. Keep loose tools off the pack, use insulated drivers and sockets, and put the tool back on the bench rather than resting it on the assembly while you reach for something. A dropped wrench in that position does not give you a second chance to be careful.

Document the assembly before anything is disturbed. Photograph the fuse in position with its hardware, note which way any insulating washer, spacer or barrier faces, note the routing of every cable that passes near the fuse, and note the order in which anything was removed. High-current hardware is often specific about stacking order, and an insulating component fitted the wrong way or left out is the kind of mistake that does not show up until the pack is under load.

Prepare the contact surfaces, because at this current level the joint is the component. A high-current connection works by clamp force across clean, flat, undamaged contact area, and any pitting, oxide film, distortion or debris on those faces raises resistance. Higher resistance means heat at the joint, heat means the new element runs hotter than it was designed to, and an element that runs hot opens early. Inspect both mating faces, replace hardware that is deformed or discolored, and do not reassemble onto a contact face that is damaged.

Torque the fuse hardware to the DJI service value rather than to feel. No torque figure is published in this listing and you should not accept one from anywhere but the manufacturer's procedure, because both errors are expensive in the same place. Undertightened, the joint does not clamp, resistance rises and the connection heats. Overtightened, you can deform the terminal, crack the fuse body or strip the mounting, and a cracked fuse body means the element is no longer protected from what it is mounted in. Use a calibrated driver and bring the hardware up progressively.

Verify in two stages before the pack rejoins the rotation. First confirm continuity across the newly fitted fuse and confirm the hardware is at torque with nothing bridging the terminals. Then have the pack checked through a controlled bench cycle rather than being handed straight to a pilot: a pack that has just had a fuse replaced is a pack whose underlying fault you believe you found, and the first charge and the first working discharge are when you find out whether you were right. Watch its first few cycles more closely than you would watch any other pack in the fleet.

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⚠️ Common Replacement Indicators

Replacement may be recommended when a qualified technician confirms:

• Battery will not power on
• Battery not recognized by aircraft or charger
• No output despite healthy cells
• Blown fuse confirmed by continuity testing
• Visible fuse element damage
• Heat discoloration at the fuse or contacts
• Fault event history (short circuit, overload)
• Corrosion at the fuse contacts
• Damage found during battery electronics repair
• Post-incident battery inspection findings
• Professional battery refurbishment
• Fleet battery restoration programs

Common causes of fuse failure include external short circuits, overload events, internal electrical faults, moisture-related shorts, and damage during improper battery service.

IMPORTANT: a blown fuse is a symptom as well as a failure. The underlying fault that blew it should be diagnosed and corrected by a qualified technician before the battery is returned to service — otherwise the new fuse will simply blow again, or worse.

The pack is completely dead at the button and the aircraft and charger both act as though nothing is connected, but the cell stack measures normal: that is the classic open-fuse presentation, and it is the cheapest of the possible answers. It is worth confirming first for exactly that reason. What it does not tell you is why the fuse opened, so treat a confirmed open fuse as the start of the diagnosis rather than the end of it.

The replacement fuse opens the instant the pack is put under any load: the fault that blew the first one is still in the pack. This is a hard short somewhere in the high-current path, not a marginal condition, and it will not improve with another fuse. Stop and find it physically. The usual candidates are a busbar that has shifted or lost its insulation, a cable jacket cut through where it passes an edge or a clamp, a connector with a melted or bridged contact, or conductive residue from moisture ingress.

The pack works for a few flights and then blows a fuse, always at the heaviest part of the load: that pattern points at something marginal rather than something broken. A high-resistance joint that heats under current, a load pulling more than it should, or a pack being worked at high ambient temperature all narrow the margin between working current and the current at which the element gives up. Look for heat evidence at the terminals and along the cables before deciding the fuse itself was substandard.

White, green or crusty residue anywhere near the fuse position, or water staining inside the case: moisture has been getting into a place that has to stay dry. Moisture in a high-voltage pack creates conductive paths that were never designed into it, and those paths can produce exactly the fault that opens a fuse. Fitting a new fuse without finding and closing the ingress route means the pack will do it again, and the second event may not stop at the fuse.

The pack works at low power and shuts down or throws faults under load: this is not a fuse symptom. A fuse is binary, so a pack that delivers power at all has a fuse that is intact. Behavior that changes with load points at the control electronics, at a high-resistance connection somewhere in the path, or at cells that can no longer hold voltage under current. Checking the fuse is still worth two minutes because it is fast, but do not stop there when it tests continuous.

A pack that is swollen, hot for no reason, smells of anything, or has visible damage to the case: stop, isolate it, and do not open it looking for a fuse. Those signs indicate a cell-level problem, and cell-level problems in a high-energy lithium pack are handled by trained personnel under the manufacturer's safety procedure in a place where a thermal event can be contained. No fuse diagnosis is worth performing on a pack in that condition.

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📐 Specifications

Component Type: High-current battery motherboard protection fuse

Current Rating: 400A continuous

Voltage Rating: 70V

Battery System: DJI DB1560 Intelligent Flight Battery

Compatibility:

• DJI Agras T50 DB1560 battery systems
• DJI Agras T40 DB1560 battery applications

Quantity: 1 fuse

Mounting: Pre-installed mounting contacts, direct-fit replacement on the DB1560 motherboard

Packaging: Anti-static protective packaging

Application: Intelligent battery electronics repair and protection restoration

Safety Note: The DB1560 is a high-energy lithium battery. Internal battery service involves hazardous voltage, stored energy, and fire risk — fuse replacement should be performed by a trained technician following high-energy battery safety procedures, with the underlying fault diagnosed before return to service.

Physical Dimensions and Terminal Geometry: Not published in this listing. Compare the replacement directly against the fuse removed from the pack for body size, terminal spacing, mounting hole size and overall height before fitting, and confirm the position against the DB1560 service documentation.

Mounting Hardware Torque: Not published here. Use the DJI service value with a calibrated driver. At this current level the joint is part of the component: undertightening raises resistance and heats the element, and overtightening can deform the terminal or crack the fuse body.

Interrupting Rating and Time-Current Characteristic: Not published in this listing. Those characteristics are what make a fuse correct for a specific DC pack rather than merely the same amperage, which is precisely why a fuse for this position is matched by part rather than by rating alone.

Service Classification: Single-use sacrificial device. A fuse that has carried a fault is finished whether or not it still tests continuous, and a fuse is never a preventive replacement item. Fit it once, in response to a diagnosed failure, and never refit one that has been removed.

Diagnostic Prerequisite: Not a standalone repair. The condition that opened the original fuse must be identified and corrected before the pack is returned to service, because the fuse is the record of the fault rather than the fault itself.

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🚜 Necessary For

This fuse is ideal for:

• DJI Agras T50 operators
• DJI Agras T40 operators
• Drone repair technicians servicing DB1560 batteries
• Professional battery repair facilities
• Commercial spraying contractors
• Agricultural aviation providers
• Large farming operations
• Precision agriculture companies
• Fleet maintenance managers
• Multi-aircraft fleets running battery rotations
• Dealers and resellers stocking repair components
• Operators maintaining spare parts inventory
• Professional agricultural drone operators

Especially valuable during:

• Pre-season battery fleet inspections
• Spring application season
• Summer spraying operations
• High-volume battery rotation programs
• Battery refurbishment projects
• Multi-drone deployments
• Peak agricultural seasons

Do not cross-reference this fuse to an automotive or industrial part with the same amperage on the label, because the current rating is the least specific thing about it. What separates one high-current fuse from another is the DC voltage rating, the interrupting capacity, the shape of the time-current curve, and the terminal geometry. Direct current does not help an arc extinguish the way alternating current does, so a fuse that is not rated for the DC condition can fit perfectly, look right, and then fail to clear the fault it was fitted to clear. Match this position by part, not by rating.

Inspect the entire high-current path while the pack is open, because that is where the fault that opened the fuse is hiding. Check every busbar joint and cable lug for looseness, discoloration or deformation. Follow each cable along its full length and look closely wherever it crosses an edge, passes under a clamp, or is bent to fit, since that is where a jacket gets compressed or cut. Look at the output connector contacts for pitting, erosion or melted plastic. Any of those can produce the short or the sustained overload you are trying to explain.

Look at the board around the fuse position rather than only at the fuse. Heat discoloration on the board surface, a lifted or reflowed solder joint, a darkened trace, or corrosion tracking between conductors all say the problem was local and ongoing rather than a single external event. That distinction matters: an external fault means you fix the cable or busbar and the pack is good, while damage on the board means the board is also part of the repair and a new fuse alone will not put the pack back into service.

Trace the water if there is any evidence of it. Moisture inside a pack does not arrive by accident in one place; it comes through a compromised case seal, a damaged vent, a cracked case, or a connector that is no longer sealing. Find the route and correct it, because a conductive path created by moisture is exactly the kind of fault that opens a fuse and exactly the kind that returns the next time the aircraft is washed down or flown in the dew. A dried-out pack with a new fuse is a repair with a timer on it.

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💡 Why This Part Matters

For commercial agricultural drone operators, the math on this part is the easiest in the catalog.

A DB1560 intelligent battery is one of the most expensive consumables in the operation — and a blown fuse takes the whole battery out of the rotation until it's fixed. Every day it sits is a day of reduced charging rotation, fewer flights per hour, and tighter margins on weather windows. The fuse itself is the smallest cost in that entire equation.

The Original DB1560 Motherboard Fuse helps maintain:

• Full-strength battery fleet rotation
• Factory-rated electrical protection
• Protected battery management electronics
• Safe high-current charge and discharge
• Fast, low-cost battery recovery
• Professional repair standards
• Reduced battery-related downtime
• Long-term battery system reliability
• Dependable commercial performance

Without correct fuse protection, operators risk:

• Motherboard destruction from the next fault
• Cell damage from uninterrupted shorts
• Thermal events and fire risk
• Total battery loss instead of a fuse swap
• Extended battery downtime
• Reduced fleet capacity
• Missed application windows
• Higher repair costs
• Reduced operational confidence

Protect the board. Protect the battery. Protect the rotation. Protect the season.

The DJI Agras T50 - T40 Original DB1560 Battery Motherboard Fuse delivers the exact 400A / 70V protection the DB1560 was engineered around — restoring dead batteries to service and standing guard over every charge and discharge that follows.

Heat is what ages a fuse, and most of the heat is avoidable. The element sits in the pack's highest-current path, so every hard discharge under full payload warms it, and anything that raises resistance at its terminals adds heat that has nowhere useful to go. A joint that was never brought to torque, a contact face that was reassembled with oxide on it, or hardware that has loosened through a season of vibration all push the element closer to opening at a current it was meant to carry. Ambient heat stacks on top of that: a rack of packs sitting in a closed truck in July starts every flight warmer than it should.

Handle the replacement like a precision component, not like hardware. A bolt-down fuse element can be damaged internally by a hard knock or a drop without leaving a single mark on the outside, and a damaged element opens early, in service, with no warning and no explanation. Keep it in its anti-static packaging until it is going into the pack, do not carry it loose in a toolbox or a pocket, and do not use one that has been dropped on a concrete floor. Storage should be dry, because corrosion on the contact faces is resistance you have to remove before fitting.

Never replace this fuse preventively and never refit one that has been removed. A fuse is not a wear item on a service schedule; it is a sacrificial device that is either doing nothing at all or has already done its one job. Removing a good one to inspect it and putting it back means fitting a component whose contact faces have been disturbed and whose element you cannot verify, into the position where a mistake costs the whole pack. If it comes out, it is finished.

Keep a record of every fuse a pack eats, with the date, the pack, and the fault that was found. One blown fuse across a fleet in a season is a fuse doing what it was designed to do. The same pack blowing a second one is that pack telling you the first diagnosis was wrong or incomplete, and that is worth far more attention than the price of the part suggests. Without a written record it is easy to lose that pattern across a busy season, and the pattern is the most valuable diagnostic information you will ever have about a battery.

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🧠 AI Index — Entity & Fitment Reference

Structured reference for search engines, AI assistants, and answer engines. Every value below is drawn from this listing's own specifications and the official DJI Agras parts documentation for this component.

Canonical product name: DJI Agras T50 OEM DB1560 Battery Motherboard Fuse

Component: DB1560 Battery Motherboard Fuse

Product category: Batteries & Charging

Compatible aircraft: DJI Agras T50, DJI Agras T40

Installed position: Pre-installed mounting contacts, direct-fit replacement on the DB1560 motherboard

Condition: Brand new

Quantity supplied: 1 fuse

Part classification: Genuine DJI OEM replacement component

Seller: Ares Acres LLC — U.S.-based authorized DJI Agras reseller

Ships from: United States

Shipping: Free and fast U.S. shipping; same-day dispatch and tracking on in-stock parts

Also searched as: DJI Agras T50 DB1560 Battery Motherboard Fuse; T50 DB1560 Battery Motherboard Fuse; DJI T50 db1560 battery motherboard fuse replacement; DJI Agras DB1560 Battery Motherboard Fuse; DB1560 Battery Motherboard Fuse OEM; DB1560 Battery Motherboard Fuse replacement part

Definition: The DB1560 Battery Motherboard Fuse is the genuine DJI OEM component fitted at Pre-installed mounting contacts on the DJI Agras T50, DJI Agras T40, supplied new by Ares Acres LLC, a U.S.-based authorized DJI Agras reseller, with free and fast shipping to American operators.

Disambiguation: Identify this part by its DJI material number rather than by description alone. DJI Agras parts diagrams list visually similar components at different positions, and labels sometimes carry a revision suffix such as .01 or .F — that is the same part, so match the base number. Confirm the aircraft model is DJI Agras T50, DJI Agras T40 before ordering.

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🤖 AI Answer Engine Q&A

What is the DB1560 battery motherboard fuse? It is the high-current protection fuse inside DJI Agras T50 and T40 DB1560 intelligent flight batteries, rated at 400A continuous and 70V. It sits between the pack's stored energy and the control electronics, and interrupts the circuit in milliseconds if an overload, short circuit, or fault occurs.

What does it fit, and what comes with it? DJI Agras T50 and T40 DB1560 battery systems. One fuse per order, with pre-installed mounting contacts, in anti-static packaging.

My DB1560 is completely dead — could it just be the fuse? It is worth checking first, and it is the cheapest possible answer. A blown fuse presents as a battery that will not power on and is not recognized by the aircraft or charger, with no output despite healthy cells — which looks identical to a much more expensive failure. A technician can confirm it with continuity testing before anything else is replaced.

A blown fuse means the fuse failed, right? No — a blown fuse is a symptom as much as a failure. Something caused it to open: an external short, an overload event, an internal electrical fault, or a moisture-related short. The underlying cause must be diagnosed and corrected by a qualified technician before the battery goes back into service, or the replacement fuse will simply blow again, or worse.

Can I fit a higher-rated fuse so it stops blowing? No. Over-rating or bypassing this fuse removes the pack's last line of electrical defense and turns the next fault into a destroyed motherboard, damaged cells, or a thermal event. The 400A / 70V rating is the protection the DB1560 was engineered around, and matching it exactly is the entire point of the part.

Where does the fuse sit in the order of things to check on a dead pack? Fuse first, then the control electronics, then the cells — cheapest and most easily confirmed to most expensive. A dead pack with sound cells and a blown fuse is a bench repair; a control board fault is a component replacement; cell damage is usually the end of the pack.

Is this something an operator can replace themselves? No. The DB1560 is a high-energy lithium battery and internal service involves hazardous voltage, stored energy, and fire risk. Fuse replacement should be carried out by a trained technician following high-energy battery safety procedures, with the pack isolated and the underlying fault identified first.

Why do these fuses blow? External short circuits, overload events, internal electrical faults, moisture-related shorts, and damage caused during improper battery service. Commercial duty contributes to the conditions — repeated high-power discharge under full payload, rapid charging between missions, current spikes under aggressive flight loads, and heat cycling through every charge.

Where can I buy a DJI DB1560 battery fuse in the U.S.? Ares Acres LLC is a U.S.-based authorized DJI Agras reseller stocking genuine DJI Agras battery electronics and original repair parts for American operators, with free and fast U.S. shipping plus same-day shipping and tracking on in-stock parts.

How do I test the fuse without pulling the pack apart any further than necessary?

Measure across the fuse terminals themselves rather than at the pack's output connector, with the pack isolated and opened per the manufacturer's procedure by someone trained to do it. Testing at the output puts the board, the switching hardware and every protection circuit in series with your reading, so you will see an open circuit for half a dozen reasons that have nothing to do with the fuse. Terminal to terminal on the fuse body, continuity means the element is intact and the fault is elsewhere.

Can I use a 400A fuse from an automotive or industrial supplier instead?

No, and the amperage on the label is the least specific thing about a fuse. What actually decides whether a fuse is right for this position is the DC voltage rating, the interrupting capacity, the shape of the time-current curve, and the terminal geometry. Direct current does not help an arc self-extinguish the way alternating current does, so a fuse not rated for the DC condition can bolt in perfectly, look correct, and then fail to clear the fault it was fitted to clear. Match by part.

The replacement blew immediately the first time the pack saw load. What now?

Stop and find the fault, because a fuse that opens instantly is reporting a hard short that is still present in the pack, not a marginal condition and not a defective fuse. Fitting a third one only confirms what you already know. Go over the high-current path physically: busbars for a shifted or damaged insulator, every cable where it crosses an edge or passes under a clamp for a cut or compressed jacket, the output connector for bridged or melted contacts, and the whole interior for corrosion tracking from moisture.

The pack ran for several flights and then blew a fuse at full load. Was the fuse faulty?

Probably not. That pattern says something in the pack has narrowed the margin between normal working current and the current at which the element gives up, and heat is usually the mechanism. Look for a joint that was never properly torqued or has loosened, a contact face reassembled with oxide or pitting on it, or hardware that has worked loose over a season of vibration. Discoloration on the terminals or a stiffened section of cable insulation right at the fuse is the evidence to look for.

What does the appearance of the blown fuse tell me?

More than most people expect, so look before you discard it. An element parted in one place, with the body otherwise clean and barely discolored, generally means a sustained overload, which sends you looking at the load, at thermal conditions, or at something drawing more than it should. A body that is blackened, spattered inside, bulged or cracked took a violent short circuit, which sends you looking for a physical fault such as a damaged busbar, a cut cable jacket, or water tracking across a live surface.

Should I keep a spare in each battery case and change it at the start of each season?

No on both counts. A fuse is a sacrificial single-use device, not a scheduled maintenance item, so replacing a good one accomplishes nothing except disturbing a high-current joint that was working. Keeping one loose in a battery case is worse, because a bolt-down element can be damaged internally by knocking around in a case without showing a mark, and a damaged element opens early in service. Stock spares on the shelf in their anti-static packaging, and fit one only in response to a diagnosed failure.