P0040
O2 Sensor Signals Swapped Bank 1 Sensor 1/Bank 2 Sensor 1
The pre-cat oxygen sensors of bank 1 and bank 2 appear to be wired the wrong way around — bank 1 reacts when bank 2 should and vice versa. Typically after recent service.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: easy
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
Common causes
Ordered by how often each turns out to be the cause.
- Sensors swapped after recent replacementCommon
- Wiring harness routed across banksPossible
Parts involved
- Oxygen Sensor B1s1
- Oxygen Sensor B2s1
- Sensor Wiring
P0040 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.