U0100 means a control module has lost communication with the engine computer over the car's internal network. It is a wiring, power or module fault rather than a mechanical one, and it usually turns up with several warning lights at once. Get it checked before driving far. Indicative UK repair cost is £60 to £800.
Modern cars run a shared data network, in most cases CAN bus, with every module talking to the others over a twisted pair of wires. U0100 is one of those modules reporting that the engine computer has gone quiet on it. The important consequence is that this code is rarely alone. Once modules stop hearing from each other they each log their own complaint, and the scan comes back with a list that looks catastrophic. The length of that list is not a measure of how serious the fault is. It is very often one wire, one earth or one failing battery.
Can I drive with U0100?
Get it checked before driving far. What makes this one urgent is unpredictability rather than damage. Depending on which module has lost contact you can lose steering assistance, anti-lock braking, stability control, or the gearbox's ability to select the right gear, and the car will not necessarily warn you which it is about to drop. A vehicle that can lose a safety system without notice is not one to take on a long journey while you think about it.
What U0100 means for your MOT
Two routes, and the first of them is the opposite of what most people expect.
- An engine lamp that does nothing. The defect wording in the DVSA MOT inspection manual is "engine MIL inoperative or indicating a malfunction", and the tester switches the ignition on specifically to watch the lamp come on and then go out. A communication fault can stop the instrument cluster ever being told to light it, so a car can fail for a warning lamp that stays dark. That is a major defect under section 8.2.1.2 for petrol cars first used on or after 1 July 2003, and 8.2.2.2 for diesels first used on or after 1 July 2008.
- Whatever else went quiet alongside it. If the anti-lock braking warning device is caught up in the same fault, that is a separate major defect under section 1.6, which fails a car both for a warning device showing a malfunction and for a warning device that is not working. The same section lists damaged wiring as a major defect on its own, and damaged wiring is the most common cause of this code in the first place.
What usually causes it
- A wiring, connector or earth fault. The commonest cause by a wide margin. A chafed CAN pair where the loom crosses a bracket, a corroded earth strap, water that has found its way into a connector, or damage left behind by previous work or an accident repair.
- A failing control module. Less common than owners tend to assume, and the most expensive outcome, because a great many modules have to be coded to the car after they are fitted.
- A weak battery or a charging fault. This belongs further up the list than its position suggests. Modules misbehave at low voltage, and a car that has been stood for a few weeks, or is on the last winter of its battery, will produce this code along with a dozen others that mean nothing.
- A fault on the network itself. One module that has failed internally can drag the whole bus down with it, which means the module doing the complaining is frequently not the module at fault.
What it costs to fix in the UK
£60 to £800. A battery, an earth strap, or a repair to a damaged section of loom sits at the bottom. Properly tracing an intermittent network fault is labour rather than parts and sits in the middle. A replacement module, coded to the car, is the top.
This is a code where paying for time rather than parts is the right instinct, which makes it worth knowing what a car diagnostic costs in the UK before you commit to anything.
These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.
What to check first
- Test the battery and the charging system before anything else. It is the cheapest possible cause, it is genuinely common, and hunting a network fault on a car with a tired battery wastes everyone's time and money.
- Go over the earths. Battery terminals, the engine-to-body strap, and the main chassis earth points. Corrosion in any of those produces symptoms that make no sense anywhere else on the car.
- Clear the codes and see which one returns first. On a car presenting a long list, clearing everything and re-reading after a short drive shows you which module noticed the problem first. That narrows the search far quicker than reading the same list again.
- Look for signs of previous electrical work. Aftermarket alarms, tow bar wiring, dash cams and stereo installations get spliced into places they have no business being. A network fault that appeared shortly after any of that has an obvious first suspect.
Related codes
- P0340, camshaft position sensor circuit. A different system, but the same diagnostic discipline: check the connector and the loom before condemning anything expensive.
U0121 covers the same loss of communication with the ABS module, P0562 covers low system voltage, which is one of the underlying causes above, and P0700 is the transmission module's equivalent flag.
Get it narrowed down before you spend
A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.
Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.