P0620 — Generator Control Circuit
Seen on: Heavy Truck
Generator Control Circuit. Common causes: Alternator control circuit fault.
Your code rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Alternator control circuit fault
- Failed alternator
- wiring fault
- ECM output issue
First moves (roadside)
- Check alternator LRC/DFM circuit
- test alternator output
- replace if needed
FAQ
Generator Control Circuit. Common causes: Alternator control circuit fault.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Alternator control circuit fault. Other possibilities include Failed alternator; wiring fault; ECM output issue.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack and more). Not a substitute for a scan-tool diagnosis on your specific truck.