OBD error code P0040 is a diagnostic trouble code (DTC). It is a generic OBD-II powertrain code as it applies to all makes and models. Though it applies to all, the repair mechanism may be different depending upon the manufacturers. OBD error code P0040 means that your vehicle’s PCM also known as the powertrain control module has detected an issue with the Oxygen sensors. It has detected that the first o2 downstream from the engine have swapped wirings. Parts Avatar offers best solution for your car’s needs. Our online parts store is open for your orders 24/7.