P0040

Review recent repair history. This code almost always follows O2 sensor replacement, exhaust work, or engine work where sensor connectors were disconnected. Identify if any recent service could have crossed the connections.

With the engine running, monitor Bank 1 Sensor 1 and Bank 2 Sensor 1 O2 voltages simultaneously. Induce a lean condition on one bank (e.g., by pulling a vacuum hose on Bank 1) and verify the correct sensor responds — if the Bank 2 sensor responds instead, the sensors are swapped.

Physically trace the wiring from each upstream O2 sensor to its connector. Verify that the sensor physically located in the Bank 1 exhaust manifold connects to the Bank 1 harness connector, and Bank 2 to Bank 2.

If sensor placement is correct, check for crossed wires at any repair splice points or aftermarket wiring modifications. The sensor signal wires may have been crossed during a repair.

After correcting the swap, clear codes, drive through two warm-up cycles, and verify fuel trims normalize on both banks (long-term fuel trim within +/- 5%).