Generic fault code
U0009 — High speed CAN communication bus (-): shorted to bus (+)
The control unit stores code U0009 when it detects this fault: “High speed CAN communication bus (-): shorted to bus (+)”.
Indicative severity: HighCommunication network
Reviewed by Daniel Morales, founder of Reprorace SL, on
What does code U0009 mean?
Control units talk to each other over buses such as CAN. A U code means one control unit has stopped hearing another: the cause may be that other unit, the wiring or the power supply, often a low battery.
- System
- U · Network and communication
- Area
- Communication network · Car system: CAN network and modules
- Type
- Generic (SAE J2012 / ISO 15031-6): the same meaning on every make
- Indicative severity
- High. Diagnose as soon as possible: it may affect safety or cause damage (engine, catalyst, brakes).
What usually causes code U0009?
- Communication network fault (bus wiring, termination resistor, gateway)
- Damaged, chafed or pinched wiring
The most likely causes come first. Only a check on the car confirms which one is yours.
What are the symptoms of code U0009?
- Several warning lights at once
- Instrument cluster or functions failing on and off
- In some cases, the car will not start
These are the usual symptoms of this kind of fault: not all of them always appear, and sometimes you notice nothing.
Can I keep driving with code U0009?
Better not, except when you really have to. This fault may affect safety or damage the engine, the catalytic converter or the brakes. See a workshop as soon as possible; if you must move the car, drive gently and only short distances. If the light flashes, the car loses power, makes unusual noises or the brakes or steering feel different, stop somewhere safe and call for assistance.
This is guidance only: only someone who checks the car can say whether it is safe to drive.
An indicative description of the code's generic meaning. Each manufacturer may give it a different meaning, conditions or tests: always follow its service information and your workshop's advice.
What you can do yourself
For this kind of fault
A flat battery, a jump start or a newly replaced battery can leave network codes: check the battery, clear the codes and read again. If they come back, the workshop will check the bus wiring and connectors.
Read the code and keep the report: with an ECO-LITE in the OBD socket you see it on your phone or computer, with the data captured when the fault happened.
Look for anything obvious: a loose connector, a disconnected hose, a fuel cap not closed properly or a low fluid level. Do not take apart anything you do not know.
Clear the code only after writing it down or after the repair. Clearing turns the light off, but it also deletes the freeze-frame data and the readiness monitors used at the inspection.
Drive normally for a few days and read the codes again. If the code comes back, the fault is still there: book the workshop and bring the report.
Read your car's fault codes on your phone
With an ECO-LITE in the OBD socket you see the codes in your account, with the data captured when the fault happened, and your workshop can check them remotely, always with your permission.
Related codes
- U0006High speed CAN communication bus (-): open circuit
- U0007High speed CAN communication bus (-): signal low
- U0008High speed CAN communication bus (-): signal high
- U0010Medium speed CAN communication bus: malfunction
- U0011Medium speed CAN communication bus: performance
- U0012Medium speed CAN communication bus (+): open circuit
U00xx codes: communication bus (wiring and signals)
In the glossary
Read more: Remote OBD diagnostics: what it is, how it works and what it can't do