P0622: Generator Field/F Terminal Circuit
P0622 means the engine computer sees an implausible signal on the F (field) wire from the alternator's voltage regulator. The car often keeps running for a while, but the alternator may not be charging properly, so the battery can go flat and you should have it checked soon.
You can drive, but get it checked soon
The code does not put the car in limp mode, but it can go along with a real charging fault. If the alternator is not charging, the battery drains and the car can stall and not restart, so keep trips short and get it tested promptly.
What it means
The alternator (called the generator in this code's name) has a regulator that controls how hard the alternator works. On many cars the regulator reports its workload back to the engine computer on the F line as a duty cycle, meaning the percentage of time the field circuit is switched on. The computer uses this to estimate how much torque the alternator draws and to raise idle speed when electrical loads are high. P0622 sets when that signal fails a plausibility test. With the ignition on and the engine off, the duty cycle should be near 0 %, and a reading of 65 % or more is a fault. With the engine running below 3000 rpm, it should be between 5 % and 99 %, and a reading of 5 % or less is a fault. Some cars handle this over a LIN bus (a simple data line) instead. Other systems split this check into P0625 (low) and P0626 (high). Some GM modules from 2023 and 2025 turn on the check engine light on the first trip, while the Saab 9-4X sets the code without a light and only when P0621 is not set.
Likely causes
- F line or alternator connector open or shortedVery likely
A broken, corroded or shorted wire or plug on the F line makes the computer read a duty cycle that does not match what the regulator is sending.
- Regulator or alternator faulty (field driver)Very likely
The part of the regulator that drives the field and reports the duty cycle can fail and send a wrong or missing signal.
- Alternator not charging (belt, brushes), duty cycle implausiblePossible
A slipping or broken belt or worn brushes (the contacts that feed the field) can leave the alternator unable to charge, which makes the duty cycle look implausible.
- Module or softwareLess likely
A fault inside the engine computer or its software can misread a normal signal, though this is the least likely cause.
What you may notice
- Battery warning lamp on
- Check engine light possible on GM from model year 2023
- Idle fluctuates or drops when large loads switch on
- Flat battery with a real charging fault
What to check, cheapest first
- 1Look at the alternator drive belt for cracks, glazing or looseness, and confirm it turns the alternator. · you can do it
- 2Inspect the alternator plug and wiring for corrosion, loose pins, chafing or damage, and check the battery terminals are clean and tight. · you can do it
- 3Read all stored codes and check whether P0621, P0625, P0626 or P0620 are also present, since they point to the same circuit. · mechanic
- 4Test battery voltage with the engine off and running to confirm whether the alternator is actually charging. · mechanic
- 5Check the F line wiring for opens, shorts to ground or power, and watch the duty cycle on a scan tool at ignition on and at running idle. · mechanic
- 6If wiring is good, replace the alternator or regulator, and consider the engine module or its software only after that. · mechanic
Related codes
Frequently asked questions
Can I drive with a P0622 code?+
Briefly, with caution. The car is not forced into limp mode, but if the alternator is not charging, the battery will run down and the car may stall.
Can a loose gas cap cause P0622?+
No. This code is about the alternator's F signal circuit and has nothing to do with the fuel system or gas cap.
Will P0622 fail an emissions test?+
The code is not flagged as emissions related, but if the check engine light is on, many tests will fail the car for that light alone. Fix the fault and clear the light first.
Can I fix P0622 myself?+
You can check the belt, connector and battery terminals. Further diagnosis usually needs a scan tool and electrical testing, and the repair typically takes about half an hour to two and a half hours.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10