P0620: Generator (alternator) control circuit
The engine computer has found a fault in the circuit it uses to control the alternator, so the battery may not be charged properly.
Updated
What P0620 means
The alternator charges the battery and powers the car while the engine runs. On many modern cars the engine computer controls how much it charges, to save fuel and look after the battery.
P0620 means the engine computer has found a fault in the circuit it uses for that control. If the alternator then stops charging, the car runs on its battery alone until it’s flat.
Read My Car shows the voltage the adapter measures at the OBD2 socket (the terminal’s ATRV command) and, where the car reports it, the battery voltage in live data.
P0622, P0625 and P0626: the field circuit
The field winding inside the alternator sets how much it charges. P0622 means the circuit that controls it has a fault, and P0625 and P0626 that the voltage in it is too low or too high. The risk is the same: the battery may not be charged properly.
P0622: Generator Field/F Terminal Circuit
The circuit that controls the alternator’s field winding, which sets how much it charges, has a fault (open, or a voltage too low or too high). The battery may not be charged properly.
Common causes: faulty alternator or its voltage regulator, wiring or connector fault between the alternator and the engine computer, fault in the engine computer’s alternator control.
P0625: Generator Field/F Terminal Circuit Low
The circuit that controls the alternator’s field winding, which sets how much it charges, has a fault (open, or a voltage too low or too high). The battery may not be charged properly.
Common causes: faulty alternator or its voltage regulator, wiring or connector fault between the alternator and the engine computer, fault in the engine computer’s alternator control.
P0626: Generator Field/F Terminal Circuit High
The circuit that controls the alternator’s field winding, which sets how much it charges, has a fault (open, or a voltage too low or too high). The battery may not be charged properly.
Common causes: faulty alternator or its voltage regulator, wiring or connector fault between the alternator and the engine computer, fault in the engine computer’s alternator control.
Can I keep driving?
Drive gently
Drive only as far as you need to and have it checked soon. If the alternator isn’t charging, the car runs on its battery until it’s flat, and then the engine can cut out, with much heavier steering and braking. Switch off what you don’t need, and if the battery warning comes on or the lights dim, don’t set off on a long trip.
General guidance based on the code alone, not on the car’s actual condition. If in doubt, stop and ask a mechanic.
Watch for the battery warning, dimming lights or electrical items that stop working: they mean the battery is running down. Switch off what you don’t need, such as the heated rear window, seat heating and the blower, and head for a garage rather than a long trip.
Symptoms
- The battery warning light on the dashboard.
- Dim or flickering lights, or slow cranking.
- Codes from several other modules, often about low voltage.
- On some cars, harsh gear changes or limp mode as the voltage drops.
Common causes
- Faulty alternator or its voltage regulator. The regulator is usually built onto the alternator; depending on the alternator, it’s replaced on its own or with it.
- Wiring or connector fault between the alternator and the engine computer. Check the plug on the alternator and the control wire for corrosion and damage.
- Fault in the engine computer’s alternator control. Rare, and suspected only after the alternator and the wiring check out. Voltage spikes from a wrongly done jump start can damage it.
Typical fixes
- Compare the voltage with the engine off and running. On many cars a charging system shows roughly 13.5 to 14.5 V with the engine running; cars with smart charging or start-stop can show less at times, so a single low reading isn’t proof of a fault.
- Check the battery clamps, the earth straps and the plug on the alternator.
- Have the charging system tested, including the alternator’s control signal. A garage can command the alternator with a tool for your make.
- Have the alternator or its regulator replaced if it doesn’t respond, or the wiring repaired.
- Have the battery tested too: after running flat it may need charging or replacing.
Questions
Can I drive with P0620?
Only as far as you need to, and have it checked soon. If the alternator isn’t charging, the car runs on its battery until it’s flat, and the engine can then cut out, with much heavier steering and braking. If the battery warning comes on or the lights dim, don’t set off on a long trip.
What is the difference between P0620 and P0562?
P0562 means the engine computer sees a low supply voltage, whatever the cause. P0620 points at the circuit that controls the alternator. They often come together: a fault in the control can stop the charging, and the voltage then drops.