What U0100 Means
Another control module in the vehicle has lost contact with the main engine control module over the internal communication network (CAN bus).
Common Causes
- Blown fuse powering the engine control module
- Corroded, damaged, or loose wiring on the CAN bus network
- Failing engine control module
- Aftermarket electronics interfering with the network
Symptoms You Might Notice
- Multiple warning lights on at once
- Car may not start or may stall unexpectedly
- Dashboard gauges may behave erratically
- Loss of power steering or other assist systems in severe cases
What to Check First
- 1
Check the fuse for the engine control module first — this is the cheapest and most common cause
- 2
Inspect battery and ground connections, since a weak connection can disrupt the whole network
- 3
Have the CAN bus wiring tested for a short or open circuit if fuses and grounds are fine
- 4
This code often needs a professional scan tool capable of network diagnostics — not a basic code reader
Frequently Asked Questions
My car won't start and shows this code — what should I check first?
Check the battery terminals and the main fuse box for a blown fuse related to the engine control module. A weak or corroded connection is one of the most common causes and the easiest to fix yourself.