DTC P1641: Mil Control Circuit: Description
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2002 GMC Savana Special, 2002 GMC Savana Camper Special, 2002 GMC Savana, 2002 Chevrolet RV Cutaway, and 2002 Chevrolet Cutaway. However, it is still accessible from the selected car via links, so may be relevant.
A dash light is illuminated by the Powertrain Control Module (PCM) when the ignition is ON with the engine OFF, or if the diagnostics have detected certain errors related to the engine performance or the engine sensor status. When the PCM is commanding the Malfunction Indicator Lamp (MIL) ON, the voltage potential of the circuit will be low, near 0 volts. When the PCM is commanding the MIL OFF, the voltage potential of the circuit will be high, near the battery voltage. The primary function of the PCM in this circuit is to supply the ground for the MIL circuit.