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U0100

fault code guide

U0100: Lost Communication With ECM / PCM 'A'

Your car's other computers have lost contact with the main engine control module. The engine will usually cut out or refuse to start until the connection is restored.

How it shows up: Multiple dashboard lights will illuminate alongside the engine management light, instrument gauges may drop to zero, and an OBD scanner will display code U0100. The engine typically cranks without starting or cuts out suddenly while driving.

Updated 9 November 2026

Can I keep driving?

Stop driving

Driving is unsafe because the engine can stall abruptly without power steering or throttle control, or leave you stranded in traffic.

What to do right now

  1. 1Pull over safely to the side of the road if the engine is running, switch on your hazard lights and switch off the ignition.
  2. 2Do not repeatedly crank the starter motor, as this will quickly exhaust the 12-volt battery.
  3. 3Check under the bonnet to make sure the battery terminals are clean and tightly clamped.
  4. 4Call for roadside assistance if the vehicle will not start or dies immediately after starting.

Likely causes

Most likely first.

  1. Likely

    Flat or failing 12-volt battery

    A weak battery causes voltage to plunge during cranking, which reboots electronic control units and drops them off the communication network.

  2. Likely

    Blown fuse or faulty main power relay

    If the main relay or fuse supplying the engine control module fails, the module loses power completely and stops broadcasting data.

  3. Possible

    Damaged CAN bus wiring or loose connector

    Corrosion, severed data lines or loose wiring plugs break the physical communication circuit between vehicle computers.

  4. Rare

    Failed engine control module

    Internal hardware failure or water ingress inside the main computer prevents it from powering on and responding to network requests.

What you can check yourself

Park somewhere safe and level, engine off, before you start.

  1. 1

    Test 12-volt battery resting voltage

    Touch a basic digital multimeter across the battery terminals with the ignition off; a healthy battery reads around 12.6 volts, whereas a reading below 12.0 volts can prevent computer networks from operating.

  2. 2

    Inspect battery terminal clamps

    Check that both battery clamps are securely tightened and cannot be moved by hand, and brush off any powdery white corrosion that might cause poor electrical contact.

  3. 3

    Check engine management fuses

    Open the under-bonnet fuse box using your owner's handbook, locate the fuses marked ECM or engine control, and pull them out to confirm the internal metal strip has not snapped.

  4. 4

    Look for visible wiring damage

    Inspect the wiring harnesses and multi-pin plugs near the engine computer for loose clips, water ingress, rub marks or damage caused by rodents.

Rough cost to fix

Rough estimate

Typical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. Total costs depend largely on whether the root cause is a simple blown relay or an internal computer failure requiring dealer-level immobiliser programming.

  • Engine control relay or fuse replacement

    Do it yourself
    €5–25£5–20 in the UK
    At a garage
    €40–110£35–95 in the UK

  • 12-volt car battery replacement

    Do it yourself
    €90–220£80–190 in the UK
    At a garage
    €130–320£110–280 in the UK

  • Wiring harness or connector repair

    Do it yourself
    Garage job
    At a garage
    €150–450£130–390 in the UK

  • Engine control module replacement and programming

    Do it yourself
    Garage job
    At a garage
    €600–1,600£500–1,400 in the UK

When to go to a garage

  • The car fails to start after confirming the battery is charged and all relevant fuses are intact.
  • You spot melted wiring, chewed harnesses or water contamination inside the main fuse box.
  • The engine cuts out unexpectedly while driving and creates an immediate danger on the road.
  • A replacement engine control module must be coded and synchronised to the car immobiliser.

Questions drivers ask

Can I drive my car with fault code U0100?

In most cases you cannot drive at all, because the car will crank without starting or cut out immediately. Even if the engine runs temporarily in a backup mode, continuing to drive is dangerous because you risk stalling in moving traffic with an abrupt loss of power steering. Keep the vehicle parked until the fault is diagnosed.

Does clearing the U0100 code fix the issue?

Clearing the code with a scan tool will not resolve the underlying issue. Code U0100 triggers because a computer has physically lost power or data communication with the main control module. If the broken wire, dead relay or faulty module remains unaddressed, the code and warning lights will reappear as soon as you turn on the ignition.

Will code U0100 cause an NCT or MOT failure?

On the UK MOT, an illuminated engine management light caused by code U0100 is an automatic fail. On the Irish NCT, an engine warning light or communication code results in an Advisory Pass on its own, provided tailpipe emissions remain within limits. In practice, if U0100 causes the engine to stall or disables speedometer gauges, the car cannot complete either test.

What is the difference between fault codes U0100 and U0101?

Code U0100 means communication has dropped out with the engine control module, which manages fuel injection, ignition and throttle response. Code U0101 indicates lost communication with the transmission control module, which controls automatic gear selection and clutch pressure. Both are data bus communication faults, but U0100 prevents the engine from running while U0101 locks the gearbox into a single gear.

Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.

The warning light you may see

Engine Management Light (Check Engine)

Service soon

Related fault codes

U0101

Lost Communication With Transmission Control Module (TCM)

Service soon

U0121

Lost Communication With Anti-Lock Brake System (ABS) Control Module

Service soon

U0155

Lost Communication With Instrument Panel Cluster (IPC) Control Module

Service soon

B1318

Battery Voltage Low (Body Control Module)

Service soon

All fault codes

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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.