P0620

What it means

P0620 is the generic code for a fault in the generator (alternator) control circuit. The engine control unit commands the alternator to regulate its output, and it has detected that the control signal or the response on that circuit is not as expected. This could be an electrical fault in the wiring, the alternator's regulator or the control unit itself.

Symptoms

• Battery or charging warning light on the dashboard
• Battery voltage too low or too high with the engine running
• Flat or weak battery, slow cranking
• Dimming headlights or erratic electrical behaviour
• Engine management light on
• Stop-start system disabled or other driver-assistance features limited

Possible causes

• Faulty alternator or its internal voltage regulatorlikely
• Damaged, chafed or corroded wiring between the control unit and the alternator control terminallikely
• Loose, corroded or damaged connector at the alternatorlikely
• Poor battery earth or engine earth connections, or a weak or failing battery affecting charging controlpossible
• Blown fuse or fault in the supply or relay feeding the charging control circuit (depends on model)possible
• Fault in the engine control unit or battery management module driving the circuitrare

How to diagnose

1. Read all stored codes and freeze frame data, noting battery voltage, engine speed and load when the fault set, and check for other charging or battery-related codes
2. Check the battery condition and measure resting voltage, then measure voltage with the engine running at idle and with electrical loads on; compare with the manufacturer's specification
3. Inspect the alternator wiring, connector and terminals for heat damage, corrosion, chafing, or water ingress
4. Check the main battery, engine and body earth connections for tightness and voltage drop
5. Check the relevant fuses and relays for the charging control circuit using the wiring diagram for the specific model
6. Test the control circuit for continuity, short to earth and short to supply between the control unit and the alternator, following the manufacturer's procedure for the engine
7. Check the alternator control signal with a suitable scope or live data, as the type of control (PWM, LIN or other) depends on the model
8. Check for any software updates or technical bulletins for the vehicle if hardware tests are all passed

How to fix

• Repair or replace damaged wiring and renew faulty connector terminals
• Clean and tighten poor earth and power connections
• Replace the alternator or its regulator if it fails testing
• Replace a faulty fuse or relay after finding why it failed
• Replace or recharge a failing battery and then recheck the charging system
• Repair or replace the control unit only after all wiring and alternator tests are confirmed good, and then carry out any required coding or adaptation

Can I keep driving?

Nearby codes

• P061FGenericInternal Control Module Throttle Actuator Controller Performance
• P0621GenericGenerator Lamp/L Terminal Circuit