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.

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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

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