U0040
- Network & Communication
- Generic (SAE)
Get it looked at now
Driving may cause further damage or leave you stranded. Arrange service promptly rather than waiting.
Communication Bus Off
This code is in our catalogue but has not yet been through editorial review, so the guidance below is limited to what our structured data supports. We publish deeper diagnostic content only after a code has been reviewed — here is why.
A control module has counted enough transmit errors to enter the bus-off state defined by ISO 11898 and stops transmitting on the bus. Multiple modules typically lose communication and several warning lamps illuminate at once.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Driving may cause further damage or leave you stranded. Arrange service promptly rather than waiting.
- Urgency
- Get it looked at now
- Will it pass emissions
- Not affected
- Check engine light
- Solid
- 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
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | CAN line shorted to supply or ground | Very common | — |
| 2 | Terminating resistor failed | Common | — |
| 3 | Gateway module routing CAN messages incorrectly | Occasional | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
- CAN Bus Wiring
- CAN Terminating Resistor
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Awaiting review
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