Codes and warning lights
What do U codes and lost communication (U0100) mean?
Codes that start with U are faults in the communication network between control units: one control unit is not receiving another's messages, or receives them with invalid data. A very common cause is low battery voltage; others are connectors, wiring or a control unit that does not start up.
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How do control units communicate?
The engine, transmission, brake and instrument cluster control units exchange data continuously over buses such as CAN. Each expects the others' messages at a fixed rate.
If a message stops arriving, the one expecting it stores a lost communication code (U01xx); if it arrives with impossible data, an invalid data code (U04xx). U00xx codes point to the bus itself.
What usually causes them?
Low voltage or voltage dips during cranking: with a weak battery some control units reset, and the others record that they stopped talking.
Connectors with moisture or corrosion, chafed wires, bad grounds, a failed or unpowered control unit and, sometimes, accessories on the bus that disturb it.
What should I check first?
Check the battery and charging before anything else. If you have replaced the battery, jump-started the car or it went flat, it is normal to find old U codes: note them, clear them and see whether they come back.
If they come back, the workshop measures the bus and the power supply of the unit that does not answer, with the manufacturer's wiring diagram.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble codes
- U0100Lost communication with ECM/PCM A
- U0001High speed CAN communication bus: malfunction
- U0121Lost communication with anti-lock brake system (ABS) control module A
- U0140Lost communication with body control module
- U0151Lost communication with restraints control module
- U0401Invalid data received from ECM/PCM A
Related symptoms
- Car battery keeps going flat: causes and what to do
- ABS or ESP warning light on: what it means and what to do
Learn more
See what your car is saying
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Frequently asked questions
Is a U code serious?
It depends on which control unit is missing: if it is the engine or the ABS, it can affect driving and safety; if it is a comfort unit, less so.
Why do they appear after replacing the battery?
Because some control units lose communication while it is disconnected. If they do not return after clearing, they do not indicate a fault.
Can an OBD reader cause U codes?
A reader that works properly, no. A faulty device connected to the bus could disturb it.