P0620: Generator Control Circuit
The engine control unit has detected a fault in the alternator control circuit.
High risk
The battery may not charge. Have it checked before longer trips.
Short answer · Can I keep driving with P0620?
Short trips only. If the alternator is not charging, the battery will go flat. Watch for the battery light and have it checked soon.
What does P0620 mean?
On many modern cars the engine control unit controls the alternator output. Code P0620 means it detected a fault in that control circuit.
The control wire, connector, the alternator regulator or the control unit can be at fault. Charging may fall back to a fixed voltage or stop.
What the car does when the code is set
The battery warning and check engine light may come on. Depending on the fault, the alternator charges at a fixed voltage or not at all. If it does not charge, the battery slowly goes flat and the engine eventually stops.
How the system works
On many modern cars the engine control unit or a separate energy management module tells the alternator how much to charge. It can reduce charging during acceleration to save fuel and increase it when braking. This works through a control wire (often a digital LIN bus or a pulse-width signal) to the regulator on the alternator. The control unit also monitors feedback from the alternator. If the control or feedback circuit fails, it stores a generator control code and the alternator may fall back to a fixed voltage or stop charging.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Faulty alternator voltage regulatorThe regulator does not respond to the control unit.
- Wiring or connector faultA damaged control wire between the control unit and the alternator.
- Worn alternatorBrushes or windings worn out.
- Software or control unit faultLess common.
How it is diagnosed
- Measure charging voltageWith the engine running, compare the voltage at the battery with the expected range.
- Check the alternator connectorLook for corrosion and damaged wires, especially on the control wire.
- Check the drive beltA slipping belt reduces charging.
- Test the alternatorA workshop tests output and regulator control with the right equipment.
Questions about P0620
Can a battery replacement cause P0620?
On some cars the battery must be registered with the control unit after replacement. Otherwise charging may be controlled incorrectly.
Can I keep driving?
Only a short distance: if the alternator is not charging, the car runs on the battery alone until it is flat and the engine stops.
Is the alternator itself always faulty?
Not always. The control wire, connector or a software issue can also set this code.