Engine and gearbox (P)
P0040: O2 Sensor Signals Swapped
This code indicates that the oxygen sensor signals for Bank 1 Sensor 1 and Bank 2 Sensor 1 are swapped. This usually happens after recent service or sensor replacement.
Checked by a licensed car technician · last reviewed 30 September 2026
Symptoms
- Check engine light on
- Poor fuel economy
- Rough idle
- Failed emissions test
Common causes
- Sensors swapped after recent replacement
- Wiring harness routed across banks
- Incorrect sensor installation
How to check it yourself
- Verify recent sensor replacements
- Inspect wiring harness routing
- Check sensor connections
Can I keep driving?
The car can still be driven, but it is advisable to fix the issue soon to avoid further problems.
General guidance, not a professional inspection. Confirm faults with measurements before buying parts, and use a qualified mechanic for brakes, steering, fuel leaks, airbags, and other safety-critical work. Terms and disclaimer