Possible causes

– Battery charge
– Faulty Generator
– Generator harness is open or shorted
– Generator circuit poor electrical connection
– Engine Control Module (ECM) controlled generator
– Faulty ECM

Tech notes

The ECM is detecting a high turn-on voltage at times. Possible causes could be the alternator or the ECM.

When is the code detected?

The Engine Control Module (ECM) has fault detection circuitry which monitors the state of the generator turn on signal circuit. If the fault detection circuit senses a voltage other than what is expected, this P0621 code will set. The voltage regulator also contains fault detection circuitry. If the regulator detects a problem, the regulator will ground the generator turn on signal circuit, pulling the voltage low. This also causes the ECM to set the P0621 code.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0621 GMC Description

The Engine Control Module (ECM) uses the generator turn on signal circuit to control the generator. A high side driver within the ECM allows the ECM to turn the generator ON and OFF. When Generator operation is desired, the ECM sends a 5 volt signal to the voltage regulator via the generator turn on signal circuit. This causes the voltage regulator to begin controlling the generator field circuit. Once the Generator is enabled by the ECM, the voltage regulator controls generator output independently of the ECM. Under certain operating conditions, the ECM can turn Off the generator by turning Off the 5 volt signal on the generator turn on signal circuit.

GMC Technical Service Bulletins for P0621 code