Generator Field/F Terminal Control Circuit
What it means
The alternator field winding sets how much the alternator charges, and many vehicles use the F terminal so the engine computer (ECU) can control the field or monitor its duty. The computer found an abnormal signal on this circuit, which can cause undercharging or overcharging.
Symptoms4
- Charge light and check engine light on.
- Low charging voltage causing a weak battery, or high voltage causing overly bright lights.
- Idle speed fluctuating when electrical loads are switched on.
- Hard starting after a period of driving.
Possible causes5
- Corroded or loose alternator connector at the F terminal.
- Broken or shorted F terminal wire.
- Worn brushes, a damaged field winding or a faulty regulator inside the alternator.
- Poor ground between the engine and the body.
- Faulty control circuit inside the engine computer (rare).
How to diagnose5
- Measure battery voltage while running with and without loads, and compare with the manufacturer specification.
- Watch the field duty or charging command on the scan tool if available, and check that it changes with load.
- Inspect the alternator connector and F terminal for corrosion.
- Check continuity of the F terminal wire and its isolation from ground and voltage sources.
- Have the alternator tested on a charging system tester.
How to fix5
- Clean and secure the alternator connector.
- Repair the broken or shorted F terminal wire.
- Repair the engine-to-body ground.
- Replace the alternator if the brushes, field winding or regulator are proven faulty.
- Clear the codes and confirm stable charging voltage and that the code does not return.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
The cost is low for a connector, wire or ground repair, and reaches the upper end when the alternator is replaced.
Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026
Frequently asked questions
Is overcharging dangerous?
Yes. High charging voltage can damage the battery, bulbs and electronic modules, so do not delay if you notice unusually bright lights.
How is P0622 different from P0621?
P0621 concerns the charge lamp or L terminal circuit, while P0622 concerns the field or F terminal circuit that relates to the charging output itself.
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What was wrong? (optional)
This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.
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