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

P0040 indicates upstream oxygen sensor signals may be swapped between bank 1 sensor 1 and bank 2 sensor 1. The banks do not match expected fuel-control response.

Code details

Generic OBD-II · P0

System: Fuel / Air
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not swap sensors blindly; prove which connector or signal is assigned to each exhaust bank.

Symptoms of P0040

Check-engine light on, sometimes with rough or uneven running.
Hesitation, surging, or a noticeable change in fuel economy.
Occasionally hard starting or a smell of unburnt fuel.

Common causes of P0040

A failing or contaminated sensor (MAF, MAP, oxygen or air-fuel sensor).
Intake or vacuum leaks, or an exhaust leak near an upstream sensor.
Wiring, connector, or fuel-delivery problems that skew the reading.

Useful checks before replacing parts

Compare bank 1 and bank 2 upstream oxygen or air-fuel sensor response while inducing a controlled rich or lean change.
Inspect sensor connector routing, harness repairs, exhaust bank identification, recent sensor replacement, and crossed connectors.
Review fuel trims, misfire data, exhaust leaks, and scan-tool bank labels before concluding the signals are swapped.

Related OBD-II codes

These nearby fuel / air entries can help compare wording and pin down the fault area.