P2278: O2 Sensor Signals Swapped Bank 1 Sensor 3/Bank 2 Sensor 3
What to do
This is almost always a wiring mistake rather than a failed part, and the fix costs nothing. Two sensors have been connected the wrong way round, which happens easily on a V engine because the leads reach both sides. If the exhaust, heat shields or sensors have been worked on recently, say so — that is when it happens. Ask for the connectors to be swapped back rather than for sensors to be replaced; new sensors plugged into the same crossed connectors behave identically.
What this code means
The module has concluded that two sensors' connections are the wrong way round. This is an installation-error code — it exists because the mistake is common enough to be worth detecting, which makes it unusual and unusually cheap to fix.
How it happens. On a V engine the two rear sensors are the same part, their leads are similar lengths, and both connectors reach both sides. Once the exhaust or the underbody heat shielding is off, there is nothing physical preventing bank 1's sensor being plugged into bank 2's harness. Nothing complains at the time; the car starts and drives.
Why it is worth a code of its own rather than being left to show up as something else. Swapped sensors do not produce a broken reading, they produce a plausible one. Each bank is judged by the other bank's measurement, and since both banks normally behave similarly the values look entirely reasonable — until the banks diverge, at which point every correction is applied to the wrong side. The distinctive signature is fuel trims on the two banks moving in opposite directions and chasing each other, each correction making the other bank worse.
Without this code, that pattern is very hard to interpret and typically leads to sensors being replaced — which changes nothing, because the new ones get plugged into the same crossed connectors.
The fix is free. Swap the connectors back. The only thing worth doing alongside is checking that nothing else was crossed during the same job.
Likely causes
Start here
Less common
- Incorrect oxygen sensor fitted · moderate
- Damaged wiring harness · moderate
Rare
- Faulty oxygen sensor · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
Sign in or create an account to add what you found about P2278.