Theory Of Operation
WARNING: This page is about a different car, the 2019 RAM 3500 HD and 2019 RAM 2500 HD. However, it is still accessible from the selected car via links, so may be relevant.
The Body Control Module (BCM) monitors the time the ignition is in the off or accessory position between key cycles and then broadcasts the signal called Ignition Off Time (IOT) via the CANC Bus. The Powertrain Control Module (PCM) saves the value from a previous key cycle into powerdown flash and if certain 'When Monitored' conditions are met compares to the current value. If the 'Set Conditions' are met, then a fault U1421 is set indicating the timer is stuck. If this fault becomes active the PCM will light the MIL light immediately. If the MIL is set and the fault goes stored the PCM turns off the MIL after 3 consecutive drive cycles.