Upstream O2 Sensors Swapped Bank To Bank
How does a P0040 code trigger the check engine light? This code will trigger the light to come on under the following condition:
The Heated Oxygen Sensor Monitor, a strategy built into the computer, determines if the Heated Oxygen Sensor (HO2S) signal response for a change in fuel trim corresponds to the correct engine bank. If a response from the Oxygen Sensor being tested is not indicated, a trouble code will be set and the MIL (Malfunction Indicator Lamp) will illuminate.
Crossed Heated Oxygen Sensor harness connectors
Wiring swapped from one Oxygen Sensor connector to the other
Wiring changed at the computer connection or in the related harness
This check engine light code is caused by swapped wiring or connectors of the upstream Heated Oxygen Sensors. This would have to be the result of a previous repair, so be sure to check the vehicle history. You may also use the “Get Help” link if you need specifications or have any other related questions. Remember to refer to an appropriate manual for specific instructions.