P2278 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. 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.

Likely causes

Symptoms

• Malfunction indicator lamp on
• Catalyst or sensor codes reported for the wrong bank

Repair estimate

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

Related codes

References: patents.google.com, law.cornell.edu.