Powertrain, generic · Computer and auxiliary outputs

P0622 Generator Field/F Terminal Circuit

Check engine lightOn
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

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).

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

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

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€30–600
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

alternatorvoltage regulatorgenerator wiringengine control module

Related codes

P0625P0626P0621P2500P0620

References: law.cornell.edu, gsi.ext.gm.com, gsi.ext.gm.com, saabwisonline.com.