CAN data bus fault

What it means

The computers in a car share information over a pair of wires called the CAN bus. These codes mean the bus itself has stopped working or is unreliable, so several computers cannot talk to each other.

Codes in this group

Bank 1 is the side of the engine that has cylinder 1. Sensor 1 is before the catalytic converter and sensor 2 is after it. More terms explained.

What you may notice

• Many warning lights at once
• Gauges dropping to zero
• Engine not starting, or gearbox in limp mode
• Faults that come and go with bumps or rain

Common causes

Most likely first:

• A weak battery or poor earth connection
• Water in a connector or module
• A damaged wire in the bus, sometimes after accident repair or accessory fitting
• One failed module dragging the whole bus down
• A badly fitted aftermarket alarm, tracker or stereo

What to check first

• Check the battery and its terminals first
• Think about what was fitted or repaired shortly before the fault began
• Tracing a bus fault needs wiring diagrams and test equipment: it is a job for an auto electrician

Have a mechanic confirm. A fault code tells you which system reported a problem, not which part has failed. The same code can have several causes, and they differ between car models. Use this page to understand the code, then have a qualified mechanic test the car before any part is replaced.

After the repair

Once the cause is fixed, the code can be cleared with a scan tool, or the warning light goes out by itself after several trips without the fault. Clearing a code without fixing the cause only hides it for a while. More about clearing codes.

Related codes

Lost communication with a control module (U0100), Invalid data received from a control module (U0401). All codes.

Written in November 2026. These are generic OBD-II codes, the same on all makes. Causes and repair steps differ between models.