What is the difference between U0100 and U0401?

U0100 means the engine control unit has gone silent — nothing is arriving at all, so the fault is usually power, earth, wiring or a dead module. U0401 means messages are arriving but are being rejected as invalid: a value outside its agreed range, a failed checksum, or a counter that has stopped moving. The second is often the more subtle problem, because everything appears connected and working, and the cause can be a software mismatch or electrical noise rather than anything visibly broken.

Can a remap cause U0401?

Yes, and it is a recognised cause. Modules agree on what values to expect from each other and within what ranges, all defined at a particular software level. A tune that increases torque beyond what the gearbox or stability control module has been told to expect, or a poorly matched software load, produces messages that the receiving module rejects as implausible. It is worth mentioning any remap when the car goes in, because it is invisible to a code reader and can otherwise cost hours of diagnostic time.

Will U0401 fail an MOT?

Yes, if it has lit a lamp, which it usually has. The engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures against it. Where stability control has been disabled too, that lamp is a separate major defect with 35,322 recorded failures. Clearing the code shortly before the test does not help, because the lamp comes back as soon as the car is driven and the readiness monitors will not have completed.

Can a bad alternator cause network communication faults?

Yes, and it is worth checking because it is easily missed. An alternator with a failed internal diode produces alternating-current ripple on top of the direct-current supply, and that puts electrical noise across the whole car. Network messages are small differences in voltage between two wires, so enough noise corrupts them and the receiving module rejects them as invalid. A garage can measure ripple with a multimeter or an oscilloscope, and it is a genuine cause of communication codes on a car where all the wiring is intact.

Network code · standard

U0401Engine control unit sending data that does not make sense

Standard definition: Invalid Data Received from ECM/PCM

Last updated 27 July 2026

Quick answer

The engine control unit is still talking, but another module has decided the information it is sending cannot be right. This is a data quality problem rather than a broken wire, which makes it a different and often more subtle fault than a lost connection.

How the car works this out

Network messages carry more than values. They carry checksums, rolling counters and agreed ranges, so a receiving module can tell whether a message arrived intact and whether the number inside it is plausible. U0401 is stored when the engine control unit's messages arrive but fail one of those tests: a value outside the agreed range, a checksum that does not match, a counter that has stopped incrementing, or a figure that contradicts what the receiving module can see for itself. The difference from U0100 matters: there, nothing arrived at all; here, something arrived and was rejected.

What you might notice

Warning lamps on for systems that depend on engine data, particularly stability control and the gearbox
Reduced power or limp mode
Harsh or unpredictable automatic gearchanges
Stability and traction control unavailable
Stop-start not working
Symptoms that come and go rather than being constant

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.

1A fault in the engine control unit or in a sensor feeding it, so the values it broadcasts are genuinely wrongCommon
How to check: Read the engine module's own fault codes. If it has stored sensor faults of its own, fix those first, because U0401 is likely to be a downstream consequence rather than a separate problem.
2A software mismatch between modules, after a partial update, a module from a different vehicle, or an engine remapCommon
How to check: Ask whether the car has been remapped or had a module replaced. Modules agree on message formats at a specific software level, and mixing levels produces exactly this fault.
3Electrical noise on the network from a poor earth, a failing alternator or a badly installed accessoryPossible
How to check: Check the earth straps for voltage drop and have the charging system checked for ripple, which is a failed diode in the alternator putting interference onto the whole electrical system.
4Low or unstable system voltage corrupting messagesPossible
5Damaged network wiring introducing errors without breaking the connection outrightPossible
6A fault in the receiving module rather than the senderLess common

What to do about it

1Read every module, not just the engine. U0401 is a complaint made by one module about another, and the useful information is usually in the codes the engine unit has stored about itself.
2Declare any remap or software work. A tune that changes the values the engine unit broadcasts, or a module replaced with one on a different software level, is a genuine and common cause and one that is invisible to a code reader.
3Have the charging system checked for ripple as well as voltage. A failing alternator diode puts electrical noise across the whole car and corrupts network messages without anything looking wrong.
4Check the earth straps between engine, body and battery. The network's signalling depends on a common reference, and a corroded earth undermines it.
5Resist replacing modules speculatively. This code names a relationship between two modules, and the fault is frequently in neither of them but in the software levels or the wiring between.

Can you keep driving?

Only to a garage

The car will normally drive, often in a reduced-power mode, and the symptoms tend to be intermittent. The concern is that systems are making decisions on information that has already been flagged as unreliable, and that stability control and the gearbox may behave unpredictably. Keep the journey short and avoid fast roads until it has been looked at.

Will it fail an MOT?

Where this code has lit the engine management lamp, that is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures. If it has taken stability control out of service, that lamp is a separate major defect with 35,322 recorded failures. There is a specific point worth making about remapped cars: a tune that causes modules to disagree about the data passing between them will keep setting this code and keep failing the test, and no amount of clearing it before the appointment will help, because the lamp returns as soon as the car is driven.

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

What is the difference between U0100 and U0401?: U0100 means the engine control unit has gone silent — nothing is arriving at all, so the fault is usually power, earth, wiring or a dead module. U0401 means messages are arriving but are being rejected as invalid: a value outside its agreed range, a failed checksum, or a counter that has stopped moving. The second is often the more subtle problem, because everything appears connected and working, and the cause can be a software mismatch or electrical noise rather than anything visibly broken.
Can a remap cause U0401?: Yes, and it is a recognised cause. Modules agree on what values to expect from each other and within what ranges, all defined at a particular software level. A tune that increases torque beyond what the gearbox or stability control module has been told to expect, or a poorly matched software load, produces messages that the receiving module rejects as implausible. It is worth mentioning any remap when the car goes in, because it is invisible to a code reader and can otherwise cost hours of diagnostic time.
Will U0401 fail an MOT?: Yes, if it has lit a lamp, which it usually has. The engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures against it. Where stability control has been disabled too, that lamp is a separate major defect with 35,322 recorded failures. Clearing the code shortly before the test does not help, because the lamp comes back as soon as the car is driven and the readiness monitors will not have completed.
Can a bad alternator cause network communication faults?: Yes, and it is worth checking because it is easily missed. An alternator with a failed internal diode produces alternating-current ripple on top of the direct-current supply, and that puts electrical noise across the whole car. Network messages are small differences in voltage between two wires, so enough noise corrupts them and the receiving module rejects them as invalid. A garage can measure ripple with a multimeter or an oscilloscope, and it is a genuine cause of communication codes on a car where all the wiring is intact.

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