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.
Medium severity. Can I keep driving?Yes, you can normally drive in moderation, but get it repaired soon: you will use more fuel and the catalytic converter can be damaged over time.
Bank 1 is the side of the engine with cylinder 1. Inline engines only have this bank. Bank 2 is the side without cylinder 1, so it only exists on V or boxer engines.
Symptoms
- Malfunction indicator lamp on
- Catalyst or sensor codes reported for the wrong bank
Most likely causes
- Sensor 3 connectors of both banks plugged in crosswise after a repair
- Harness routed to the wrong side or pins swapped in a connector
- Wrong harness, ECM or calibration after a replacement
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: easy
- It affects emissions.
What each character of P2278 means
- P: Powertrain: engine and transmission
- 2: Generic code: same on every make
- 2: Fuel injection
- 78: Specific fault in that system
Frequently asked questions
Can I keep driving with code P2278?
Yes, you can normally drive in moderation, but get it repaired soon: you will use more fuel and the catalytic converter can be damaged over time.
What causes code P2278?
The most common causes are: sensor 3 connectors of both banks plugged in crosswise after a repair; harness routed to the wrong side or pins swapped in a connector; wrong harness, ECM or calibration after a replacement.
Related codes
- P0040O2 Sensor Signals Swapped Bank 1 Sensor 1/Bank 2 Sensor 1
- P0041O2 Sensor Signals Swapped Bank 1 Sensor 2/Bank 2 Sensor 2
- P2274O2 Sensor Signal Biased/Stuck Lean (Bank 1, Sensor 3)
- P2276O2 Sensor Signal Biased/Stuck Lean (Bank 2, Sensor 3)
- P0142O2 Sensor Circuit Bank 1 Sensor 3
- P0162O2 Sensor Circuit Bank 2 Sensor 3
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.