P2278
O2 Sensor Signals Swapped Bank 1 Sensor 3/Bank 2 Sensor 3
Fuel & air metering (O2 sensors)
Generic OBD-II
Medium
What it means
P2278 means the ECM has detected that the Bank 1 and Bank 2 Sensor 3 oxygen sensors appear to be reading each other's signals — their inputs are swapped. It most often follows recent exhaust or sensor work where connectors were crossed. The engine usually still runs, but emissions monitoring and fuel trims are affected until the sensors are correctly connected.
Medium
Can I keep driving? Usually yes — it mainly upsets emissions monitoring, though fuel trims can drift; fix it before an emissions test.
Common symptoms
What you'll notice
- Check-engine light on
- May idle or run slightly rough
- Engine surge and possibly higher fuel consumption
- Fails an emissions readiness check
Common causes
Most likely first
- Bank 1 and Bank 2 sensor-3 connectors plugged into the wrong sensors after recent serviceLikely
- Crossed or mis-routed O2 sensor wiring harnessCommon
- Faulty or contaminated O2 sensor giving an inverted/implausible signalCommon
- Connector corrosion or poor electrical connectionLess often
How to diagnose & fix
Work through it in order
- Confirm which sensors are Bank 1 Sensor 3 and Bank 2 Sensor 3 for your engine.
- Verify each is plugged into its correct connector — correct any that were swapped during recent work.
- Inspect harness routing and connectors for crossed wires or corrosion.
- If wiring is correct, test the suspect sensor's response and replace it if faulty.
- Clear the code and re-test on a drive cycle.
FAQ
Quick answers
Is it safe to drive with P2278?+
Yes for short trips — the engine runs, though fuel trims may drift and it will affect emissions monitoring. Fix it before an emissions test.
Can I just clear P2278?+
It returns until the sensors are correctly connected or the faulty sensor is replaced. This code often appears right after O2 sensor or exhaust work.
Related codes
Often seen with P2278
Read & clear P2278 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX