P0620 — Generator Control Circuit
What it means: Generator Control Circuit.
How serious / can you keep driving: Serious. Power derate likely — plan to stop soon.
Check first: Check alternator LRC/DFM circuit
Don't risk it — look up listed providers near you and give them this code when you call.
Listed providers nearest Indianapolis, IN
Closest first. Tell them P0620 and what the truck is doing. RoadService.app lists these providers and does not employ them or carry out the repair.
- 0.0 mi away — based in Indianapolis, IN · (317) 789-5002
- 0.0 mi away — based in Indianapolis, IN · (317) 300-7000
- 0.0 mi away — based in Indianapolis, IN · (317) 646-8208
Seen on: Heavy Truck
What P0620 means
Generator Control Circuit. Common causes: Alternator control circuit fault.
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.
Related codes
Codes that commonly appear alongside P0620, or share the same subsystem.
- ECU Power Supply — Intermittent / Erratic (SPN 627 FMI 2)
- Battery / Charging Voltage — Low (SPN 168 FMI 18)
- Engine Controller (ECM) — Internal Fault (SPN 629 FMI 12)
- Battery voltage low during crank (SPN 168 FMI 17)
- J1939 proprietary datalink lost (SPN 625 FMI 9)
- J1939 Data Link — Communication Error (SPN 639 FMI 2)
- High Speed CAN Bus Fault (U0001)
- Lost Communication with ECM/PCM (U0100)
Same symptom: Charging and voltage
Other codes that trace back to voltage — one bad ground can set half a dozen of them at once.
- System Voltage Malfunction (P0560) · Electrical, Charging & Network
- System Voltage Low (P0562) · Electrical, Charging & Network
- System Voltage High (P0563) · Electrical, Charging & Network
- Starter Relay Circuit High (P0619) · Electrical, Charging & Network
- Generator Field F Terminal Circuit (P0622) · Electrical, Charging & Network
- Generator Field Terminal Circuit Low (P0625) · Electrical, Charging & Network
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.