The PCM controls generator charging by supplying the generator a control voltage of 5 volts whenever the ignition is turned ON or the engine is running. This control voltage is necessary for the generator to charge once the engine has started and the generator begins to spin. With the ignition ON, engine OFF, the generator will ground the control voltage through a pull-down resistor. The voltage on circuit 25 with the circuit intact will read between 1-1.5 volts. When the engine is started and the generator begins to charge, the generator will open the circuit, still using the 5-volt control voltage, signaling to the PCM that the generator is charging. DTC P0621 sets when the engine is running and the voltage at the PCM generator L-terminal output is low indicating the generator is not charging or circuit 25 is shorted to ground. If this condition occurs, the PCM will send a message to the I/P cluster over the Class II link to turn ON the charge (battery) telltale.

DTC P0621 will set if the generator L-terminal output voltage at the PCM is less than 4 volts when:

DTC P0621 diagnostic runs continuously with the engine running.

DTC P0621 is a type D DTC.

With the ignition ON, engine OFF, the voltage measured with a DVOM on circuit 25 at the generator with the connector disconnected should read near 5 volts. The voltage may rise to around 5.8 volts with the engine running.

An intermittent short to ground on circuit 25 will set this DTC. An open or short to voltage on circuit 25 will NOT.

A generator that is not charging will pull circuit 25, through resistance, to ground and set this DTC. The voltage on circuit 25 with the circuit intact will read between 1-1.5 volts when this condition occurs.