What P0620 means
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.
How the engine computer detects it
P0620 is the general control circuit code. On LIN alternators it stands for disturbed control or communication: a manufacturer table published via NHTSA (gateway module with LIN alternator) lists it with the battery lamp on and stop-start disabled, alongside P065B for range/performance. Saab's 9-3 diesel uses it for the L line – open between module and alternator for more than 6 s, or a charging signal in an implausible state. Ford's 6.8 L V10 reports it with or without the check engine light depending on the variant. GM does not use P0620 and reports L and F line faults as P0621/P0622.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Battery warning lamp on
- Battery not or incorrectly charged, starting problems
- Stop-start not working
- Sometimes no noticeable symptoms
Common causes
- Connector or wire between module and alternator (corrosion, break, chafing) Most likely
- Alternator or regulator faulty, including its LIN interface Most likely
- Battery flat or faulty, jump start or charger connected Possible
- Loose or worn drive belt Less likely
- Engine module or gateway (software, driver) Less likely
How to diagnose P0620
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: alternator, voltage regulator, generator wiring, engine control module.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 6
- Computer and output circuits
- 20
- Specific fault
Questions drivers ask
Can I drive with code P0620?
Only for a short distance. When the charging system isn't working right, the car may run on battery power alone and can stall, sometimes taking power steering with it. Turn off accessories you don't need and head straight to a shop.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.