OBD-II code · Network
U0040 Communication Bus Off
A control module has counted enough transmit errors to enter the bus-off state defined by ISO 11898 and stops transmitting on the bus. Multiple modules typically lose communication and several warning lamps illuminate at once.
Is it safe to drive with U0040?
The car may go into limp mode (reduced power). Get it diagnosed before a long trip.
Common causes
- Most likelyCAN line shorted to supply or ground
- PossibleTerminating resistor failed
- Less likelyGateway module routing CAN messages incorrectly
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
Repair at a glance
- Difficulty
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 1–6 h
- Emissions test
- Usually unaffected
- Parts involved
- Can Bus Wiring, Can Terminating Resistor
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.