U0091 reports the (-) line shorted to the (+) line on vehicle communication bus G. SAE J2012 gives each vehicle bus a block of codes: the bus itself, performance, then open, low and high for the (+) and the (-) line and a short between them; which physical network carries which letter is set by the manufacturer. On CAN networks (+) normally stands for CAN-High and (-) for CAN-Low. Modules on the affected bus may stop exchanging data, so lost-communication codes and warning lights in several systems usually appear together.
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
- Several warning lights at once
- Lost-communication codes stored in other modules
- Functions of modules on this bus inoperative
- Scan tool cannot reach some modules
Most likely causes
- Bus wires shorted together, e.g. crushed harness
- Water ingress in a bus connector
- Failed bus transceiver in a module
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 U0091 means
- U: Communication network
- 0: Generic code: same on every make
- 0: Subsystem depends on the code range
- 91: Specific fault in that system
Frequently asked questions
Can I keep driving with code U0091?
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 U0091?
The most common causes are: bus wires shorted together, e.g. crushed harness; water ingress in a bus connector; failed bus transceiver in a module.
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.