P2278 code meaning
O2 Sensor Signals Swapped Bank 1 Sensor 3/Bank 2 Sensor 3
Powertrain system Generic code P22xx codes Check engine light
What P2278 means
P2278 reports that the third oxygen sensors of the two banks are connected the wrong way round, so the ECM reads the bank 1 sensor as if it sat on bank 2 and vice versa. The check changes the exhaust of one bank only, for example by cutting its injectors or feeding air into one exhaust line, and watches which sensor answers. A DaimlerChrysler patent describes this test and the risk for control sensors: crossed sensors push one bank leaner and the other richer until both controllers hit their limits. At the third position the effect is milder, since these sensors mainly watch the catalysts, but faults would be blamed on the wrong bank. No manufacturer documents a test for this code publicly; CARB requires usable secondary sensor output. The usual cause is crossed connectors after repairs.
P2278 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyEasy, DIY possible
- Typical labor0.2 to 1 hours
- Clears after repairAfter a drive cycle
Symptoms of P2278
- Malfunction indicator lamp on
- Catalyst or sensor codes reported for the wrong bank
Common causes of P2278
- Most likelySensor 3 connectors of both banks plugged in crosswise after a repair
- PossibleHarness routed to the wrong side or pins swapped in a connector
- Less likelyWrong harness, ECM or calibration after a replacement
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Oxygen sensor downstreamOxygen sensor wiringOxygen sensor connector
What each part of P2278 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 2 | Area: fuel and air metering (injector circuit). |
| 78 | The exact fault in that area. |
What to do next
- Note P2278 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P2278
What does P2278 mean?
P2278 means "O2 Sensor Signals Swapped Bank 1 Sensor 3/Bank 2 Sensor 3". P2278 reports that the third oxygen sensors of the two banks are connected the wrong way round, so the ECM reads the bank 1 sensor as if it sat on bank 2 and vice versa. It is a generic code, so it means the same on every make.
Is it safe to drive with P2278?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P2278 fail an emissions test?
Yes, it can. P2278 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P2278 to fix?
Difficulty: easy. A typical repair takes 0.2 to 1 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P2278 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.