U0100–U0115 Lost Communication With ECM/PCM A

Which code is which

Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.

The definition

U0100: Lost Communication With ECM/PCM A.

U0100 is a network code (communication between control modules), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.

What to check first

Cheapest and fastest checks first:

1. Check power and earth at the module first - a module with no supply cannot answer.
2. Check the two data-bus wires for continuity and for a short between them.
3. Scan the other modules: if several report the same module missing, the fault is at that module rather than on the bus.

What actually causes this on a VAG

Another control unit - commonly the gearbox, ABS or the instrument cluster - has reported that it can no longer hear the engine control module (J623 in VAG's own numbering) on the drivetrain network. It does not mean the engine ECU has failed. On VAG cars this is one of the most commonly stored codes there is, because anything that briefly upsets the car's electrical supply makes several modules lose sight of each other and log it.

Most likely causes, in order

1. Weak battery, poor earth or a jump start. By far the most common reason, and the cheapest. Low or unstable voltage makes modules reset and drop off the network. If the code is stored alongside a spread of unrelated communication and voltage faults in several modules, and the car otherwise drives normally, have the battery, charging and earth straps tested before anything else.
2. Water or corrosion at the engine ECU. On many VAG models the engine ECU sits in the box at the base of the windscreen. Blocked scuttle drains fill that area, and water gets into the ECU plugs and the module itself. Lift the cover and check for standing water, damp foam, rust or green pins before condemning anything electronic.
3. Damaged drivetrain CAN wiring or a loose plug. Look at the ECU connectors, the loom where it crosses the engine bay, and any point a loom has been disturbed by recent work. Also check the relevant fuses and the ECU's supply and earth.
4. A genuinely failed engine ECU. Real but uncommon, and typically accompanied by a non-start with no communication to the ECU at all from any scan tool. Note that a replacement ECU must be coded and matched to the immobiliser, so this is not a part to swap speculatively.

Worth knowing: Check whether the fault is current or just stored history, and write down every code in every module before you clear anything - clearing throws away the freeze-frame data that tells you what the car was doing when it happened. If the car starts, drives and scans normally, a single stored U0100 is very often the fingerprint of a flat battery, a jump start or a battery disconnect: clear it, drive for a few days, and only investigate further if it comes back. If instead it returns straight away, or the car is in limp mode, or the ABS and ESP lights are on alongside it, stop treating it as history - that combination points at a live supply, earth or wiring problem and wants finding properly rather than clearing again.

Common questions

What does U0100 mean?

U0100 means Lost Communication With ECM/PCM A. This module has stopped talking on the car's data network, so nothing else can hear it.

Can I drive with U0100?

The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.

Related reading

• Several warning lights on at once: usually one cause, not many
• Do you actually need a VAG-specific scan tool?
• Key not recognised or car won't unlock: VW and Audi
• Why the same fault code has completely different causes