P0040 O2 Sensor Signals Swapped Bank 1 Sensor 1/Bank 2 Sensor 1

On engines with two exhaust banks, the control module checks whether the upstream oxygen sensors are assigned to the right bank. It changes the mixture on one bank only, actively or as part of normal bank-wise control, and watches which sensor responds. If the sensor that should sit on the other bank reacts, the signals are swapped. This logic is documented for the manufacturer-specific predecessor codes; the usual cause is connectors plugged in crosswise after a repair. If the swap remains, each bank is controlled by the other bank's signal, so fuel trims drift towards their limits.

Likely causes

Symptoms

• Check engine light on
• Fuel trim and mixture codes possible
• Unstable idle or higher consumption possible

Repair estimate

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

Parts involved

Related codes

References: v8sho.com, law.cornell.edu, en.wikipedia.org.