The Transmission Control Module (TCM) is often integrated in a single housing with the Powertrain Control Module (PCM). But even so, the automatic transmission function controller is turned on separately. A fuse or relay and its respective links are included in the circuit. Although the PCM and TCM are usually powered by the same relay, each has an individual circuit.
Once the ignition switch is activated, the vehicle’s main computer performs a self-test of all controllers. In the event that the TCM power input signal is not within the acceptable range, the DTC P0880 OBD2 is set.