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. 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.
Medium severity. Can I keep driving?You can go straight to a repair shop if there are no other warnings, watching the battery warning light and avoiding unnecessary electrical loads. If the battery runs down, the engine may stop and you may lose power steering.
Symptoms
- Battery warning lamp on
- Battery not or incorrectly charged, starting problems
- Stop-start not working
- Sometimes no noticeable symptoms
Most likely causes
- Connector or wire between module and alternator (corrosion, break, chafing)
- Alternator or regulator faulty, including its LIN interface
- Battery flat or faulty, jump start or charger connected
- Loose or worn drive belt
- Engine module or gateway (software, driver)
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 P0620 means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 6: Control modules and outputs
- 20: Specific fault in that system
Frequently asked questions
Can I keep driving with code P0620?
You can go straight to a repair shop if there are no other warnings, watching the battery warning light and avoiding unnecessary electrical loads. If the battery runs down, the engine may stop and you may lose power steering.
What causes code P0620?
The most common causes are: connector or wire between module and alternator (corrosion, break, chafing); alternator or regulator faulty, including its LIN interface; battery flat or faulty, jump start or charger connected.
Related codes
- P0621Generator Lamp/L Terminal Circuit
- P0622Generator Field/F Terminal Circuit
- P065BGenerator A Control Circuit Range/Performance
- P0562System Voltage Low
- P0563System Voltage High
- P2500Generator Lamp/L-Terminal Circuit Low
Sources
- law.cornell.edu: 13 CCR 1968.2
- static.nhtsa.gov: MC 10229413 9999
- saabwisonline.com: p0620 04
- saabwisonline.com: p0620 0f
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.