The alternator regulator is linked to the engine module by an L (turn-on) line, an F (field) line reporting field load as a duty cycle, or on newer cars a LIN bus. P0622 checks the F feedback: its duty cycle equals the field on-time, from which the module calculates alternator torque and raises idle under load. Two plausibility tests: with ignition on and engine off the duty cycle must be near 0 % – a fault at 65 % or more; with the engine running (below 3000 rpm) it must be 5–99 % – a fault at 5 % or less. GM's 2023 and 2025 modules use 5 s each and, unusually, set the check engine light in the first trip; the Saab 9-4X uses 5 s and 15 s without it and only when P0621 is not set. Other systems split the same check into P0625 (low) and P0626 (high).
Medium severity. Can I keep driving?If the battery warning light comes on, you could get stranded. Turn off electrical accessories and go straight to a repair shop.
Symptoms
- 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
Most likely causes
- F line or alternator connector open or shorted
- Regulator or alternator faulty (field driver)
- Alternator not charging (belt, brushes), duty cycle implausible
- Module or software
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
What each character of P0622 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 6: Control modules and outputs
- 22: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0622?
If the battery warning light comes on, you could get stranded. Turn off electrical accessories and go straight to a repair shop.
What causes code P0622?
The most common causes are: f line or alternator connector open or shorted; regulator or alternator faulty (field driver); alternator not charging (belt, brushes), duty cycle implausible.
Related codes
- P0625Generator Field/F Terminal Circuit Low
- P0626Generator Field/F Terminal Circuit High
- P0621Generator Lamp/L Terminal Circuit
- P2500Generator Lamp/L-Terminal Circuit Low
- P0620Generator Control Circuit
Sources
- law.cornell.edu: 13 CCR 1968.2
- gsi.ext.gm.com: 25OBDG03A
- gsi.ext.gm.com: 23OBDG03A Part2
- saabwisonline.com: dtc p0622
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.