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
– Faulty Generator
– Generator harness is open or shorted
– Generator circuit poor electrical connection
– Engine Control Module (ECM) controlled generator
– Faulty ECM
Tech description
The ECM is detecting a high turn-on voltage at times. Possible causes could be the alternator or the ECM.
When is the obd code mean?
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 OBD Code Description