Can a flat battery cause U0100?

Yes, and it is the most common explanation of all. When system voltage falls low enough — during a long crank, a jump start, or simply a battery that has gone flat overnight — modules drop off the communication network briefly. Any module that was waiting to hear from the engine control unit records that it went silent, and stores U0100. Once the battery is charged everything works normally and the code sits in memory as history. That is why establishing whether the code is current or stored is the first question to ask.

Does U0100 mean the ECU has failed?

Not usually, and it is worth resisting that conclusion because a replacement engine control unit is among the most expensive parts on a car and needs programming and immobiliser coding on top. U0100 only says the unit went quiet. A blown fuse, a failed main relay, a corroded connector, a poor earth or low battery voltage will all silence a perfectly healthy unit. Confirming that it has power on every supply pin and a good earth is a short job and should always come before condemning it.

Why is my rev counter reading zero but the engine is running?

Because on a modern car the rev counter is not connected to the engine. The instrument cluster displays a number that the engine control unit broadcasts over the network, and if those messages stop arriving the needle falls to zero even though the engine is turning perfectly well. It is a communication symptom rather than an engine one, and it is a useful clue that points at U0100 rather than at anything mechanical.

Network code · standard

U0100No communication with the engine control unit

Standard definition: Lost Communication with ECM/PCM "A"

Last updated 27 July 2026

Quick answer

Other modules in the car have stopped hearing from the engine control unit. Sometimes the engine unit has genuinely failed, but very often it is a power supply, an earth, or a battery that went flat, and the code is a leftover rather than a live fault.

How the car works this out

The engine control unit broadcasts a stream of messages that the gearbox, instruments, stability control and body modules all depend on — engine speed, torque, coolant temperature and much else. Each of those modules knows how often to expect them. U0100 is stored by a receiving module when the expected messages stop arriving for longer than a set period. It is important to note who is complaining: this code is a report from another module about the engine unit's silence, not a report from the engine unit about itself.

What you might notice

Engine cranks but will not start
Several warning lamps on at once
Rev counter reading zero while the engine runs
Automatic gearbox stuck in a single gear
A scan tool unable to connect to the engine module specifically, while other modules respond
Everything normal, with the code stored historically after a flat battery or a jump start

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1Low battery voltage, a flat battery or a jump start, which drops modules off the network and stores the code without anything being brokenCommon
How to check: Ask whether the code is current or historic, and have the battery load tested. A historic U0100 after a flat battery usually needs nothing more than clearing.
2A blown fuse or a failed relay supplying the engine control unitCommon
How to check: Check the engine management fuses and the main relay. It is quick, it is cheap, and it is a genuine cause.
3A corroded or water-damaged engine control unit connector, particularly on units mounted in the scuttle where leaves and water collectCommon
How to check: Look for water in the scuttle area and green corrosion on the pins. Blocked scuttle drains are a recognised killer of engine control units.
4A poor earth between the engine, body and batteryPossible
5Damaged network wiring between the engine unit and the rest of the carPossible
6A failed engine control unit, which is the expensive answer and should be the last one reachedPossible
How to check: Confirm the unit has power on all its supply pins and a good earth before condemning it. Replacement units almost always need programming and immobiliser coding to the car.

What to do about it

1Establish whether the code is live or historic before spending anything. A stored U0100 from a flat battery two months ago is not a fault, and clearing it is the whole job.
2Have the battery load tested and the charging system checked. Low voltage is the commonest reason for this code by a wide margin.
3Check the fuses and the main relay for the engine management system.
4Inspect the engine control unit connector and the area around it for water, especially if the unit lives in the scuttle below the windscreen.
5Confirm power and earth at the unit before agreeing to a replacement. A new engine control unit is one of the dearest parts on the car and needs coding to the vehicle and its immobiliser.

Can you keep driving?

If the code is live, the car will usually not start, and if it does start it may cut out at any moment without warning, taking the power steering assistance and the brake servo with it. If the code is historic and everything works normally, there is nothing to worry about — but that is a judgement to make with a scan tool rather than by assumption.

Will it fail an MOT?

How this affects the test depends entirely on whether the fault is live. A car that will not start cannot be tested at all. A car that runs with the engine management lamp on fails on that lamp, which is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures. And if the loss of engine data has left other modules unhappy, their lamps are separate major defects too. There is a further trap: whatever cleared this code — a flat battery, a disconnection, a repair — also reset the readiness monitors, and on many cars the emissions check cannot be completed until they have run again.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Can a flat battery cause U0100?: Yes, and it is the most common explanation of all. When system voltage falls low enough — during a long crank, a jump start, or simply a battery that has gone flat overnight — modules drop off the communication network briefly. Any module that was waiting to hear from the engine control unit records that it went silent, and stores U0100. Once the battery is charged everything works normally and the code sits in memory as history. That is why establishing whether the code is current or stored is the first question to ask.
Does U0100 mean the ECU has failed?: Not usually, and it is worth resisting that conclusion because a replacement engine control unit is among the most expensive parts on a car and needs programming and immobiliser coding on top. U0100 only says the unit went quiet. A blown fuse, a failed main relay, a corroded connector, a poor earth or low battery voltage will all silence a perfectly healthy unit. Confirming that it has power on every supply pin and a good earth is a short job and should always come before condemning it.
Will U0100 fail an MOT?: If it is live, yes, and possibly several times. A car that will not start cannot be tested at all. One that runs with the engine management lamp on fails on that lamp — a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures. Other modules deprived of engine data may light their own lamps, each a separate major defect. If the code is historic and everything works, it does not affect the test.
Why is my rev counter reading zero but the engine is running?: Because on a modern car the rev counter is not connected to the engine. The instrument cluster displays a number that the engine control unit broadcasts over the network, and if those messages stop arriving the needle falls to zero even though the engine is turning perfectly well. It is a communication symptom rather than an engine one, and it is a useful clue that points at U0100 rather than at anything mechanical.

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