Fault codes › Network › Generic (SAE standard) › communication between control modules
U0100 Lost communication with engine control module
Urgent Risk of damage or of being stranded. Stop, or get it seen, now.
What does U0100 mean?
U0100 is a generic network code, defined by the SAE standard, so it means the same thing on every make and model: lost communication with engine control module. It belongs to the communication between control modules group of codes.
A control module stores the code when something it watches stays outside what it expects for long enough. On its own the code says which system complained, not which part has failed — the causes below, most likely first, are where to start.
Is it safe to drive with U0100?
Urgent. Have it diagnosed before driving — a dropping bus can disable safety systems.
Common causes of U0100
- CAN bus wiring fault.
- Failed module.
- Power or earth fault at the module.
Symptoms
- Multiple warning lights.
- Refusal to start.
- Instruments dead.
What to check first
- Bus voltages.
- Module power and earth.
- Connector corrosion.
Can I just clear U0100?
You can, and the light goes out — but a fault that is still there sets the code again, often within a drive or two. Clearing also erases the freeze frame (what the engine was doing when the code was stored) and resets the emissions readiness monitors, so an MOT, smog or inspection test will wait until they have run again. Fix the cause first, then clear it, then watch whether it returns.
Read and clear U0100 from your iPhone with OBDTrip
Plug a Bluetooth OBD-II adapter into the car and OBDTrip reads every stored, pending and permanent code, explains each one in plain English like this, shows what the engine was doing when it was set, and clears the check engine light once the fault is fixed. It keeps a history of when each code comes back, and records every drive on its own from then on.
Needs a Bluetooth Low Energy adapter such as the Vgate iCar Pro — which adapters work with iPhone. Reading and clearing codes is part of an OBDTrip subscription.
Related codes
- U0101 Lost communication with transmission control module
- U0121 Lost communication with ABS control module
General guidance from OBDTrip's fault-code catalogue, not a diagnosis of your car. When a code points to brakes, steering, overheating or a flashing check engine light, have the car looked at before driving on.