CAN-High and CAN-Low are shorted together. The differential signal collapses to zero and the bus controller can no longer separate dominant from recessive bits, so all communication stops.
Medium severity. Can I keep driving?It depends on which control module has dropped out. If it affects the engine, brakes, or steering, do not continue your trip: call for roadside assistance.
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
Most likely causes
- CAN-H and CAN-L shorted together
- Pinched harness behind a module
- Gateway module routing CAN messages incorrectly
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: hard
- It can trigger limp mode.
What each character of U0009 means
- U: Communication network
- 0: Generic code: same on every make
- 0: Subsystem depends on the code range
- 09: Specific fault in that system
Frequently asked questions
Can I keep driving with code U0009?
It depends on which control module has dropped out. If it affects the engine, brakes, or steering, do not continue your trip: call for roadside assistance.
What causes code U0009?
The most common causes are: cAN-H and CAN-L shorted together; pinched harness behind a module; gateway module routing CAN messages incorrectly.
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.