Several things can cause this code:
* Faulty Generator Control Circuit Wiring: This is a common culprit. Look for broken, frayed, shorted, or corroded wires in the wiring harness leading to and from the alternator. This includes the ground wire.
* Bad Generator Control Module (if equipped): Some vehicles have a separate module controlling the alternator. If your Impala has one, it could be faulty.
* Faulty Alternator: While the code doesn't *directly* point to the alternator, a failing alternator can sometimes disrupt the control circuit and trigger the code. Testing the alternator's output is essential.
* PCM Issues: Though less likely, a problem with the PCM itself could also cause the P0620 code. This is usually diagnosed last after other possibilities are ruled out.
* Faulty Connections: Loose or corroded connections at the alternator, PCM, or any point in the wiring harness can interrupt the circuit.
Troubleshooting Steps:
1. Visual Inspection: Thoroughly inspect all wiring related to the alternator for damage, corrosion, or loose connections. Pay close attention to the connection points at both the alternator and the PCM.
2. Voltage Testing: Test the voltage at the alternator's output while the engine is running. It should be around 13.5-14.5 volts. A low voltage reading suggests a charging system problem.
3. Wiring Continuity Test: Using a multimeter, check the continuity of the wiring in the generator control circuit. This requires a wiring diagram to identify the specific wires.
4. Alternator Testing: Have the alternator tested at an auto parts store to ensure it's charging properly.
5. PCM Scan: A more advanced scan tool might provide more detailed information about the specific fault within the generator control circuit.
If you're not comfortable performing these tests yourself, it's best to take your Impala to a qualified mechanic for diagnosis and repair. Improperly diagnosing and repairing electrical problems can lead to further damage.
