P0040 - Upstream Oxygen Sensors Swapped From Bank To Bank

The PCM sees the oxygen-sensor signals assigned to the wrong bank, usually after sensor replacement or harness cross-connection.

Code Overview

The PCM sees the oxygen-sensor signals assigned to the wrong bank, usually after sensor replacement or harness cross-connection.

Common Causes

• Sensor connectors swapped during service
• Harness routing error
• Incorrect sensor installation

Diagnostic Steps

1. Trace the upstream and downstream sensor connectors bank to bank
2. Verify sensor connectors were not crossed during exhaust service
3. Confirm the scan-tool bank labeling matches the installed sensors

Likely Repairs & Cost Guides

Use test results to confirm the fault before pricing the repair.

Related OBD Codes

Browse the closest code-family pages when the same fault pattern often shows up nearby.

Next Steps

Move from the code to a diagnosis path, then into repair pricing once the fault is confirmed.