OBD-II code · Network
U0004 High Speed CAN Communication Bus (+) Low
The CAN-High signal stays below its valid voltage window. Common cause is a short to ground on the CAN-H wire or a transceiver that pulls the line down. The bus stops working until the offending node is isolated.
Is it safe to drive with U0004?
The car may go into limp mode (reduced power). Get it diagnosed before a long trip.
Common causes
- Most likelyCAN-High shorted to ground
- PossibleInternal transceiver failure on a node
- 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
- 1–5 h
- Emissions test
- Usually unaffected
- Parts involved
- Can High Wire, Can Transceiver
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.