Fault code P0621 – a step-by-step guide to diagnosing and fixing it

Error code P0621 is displayed and the check engine light is on - what does this mean?

The OBD system detects a fault in the alternator control circuit "L", which is responsible for the proper functioning of the alternator. This DTC P0621 indicates a problem with the voltage regulation or communication between the control unit and the alternator, which can be interpreted slightly differently by different manufacturers.

What causes can this error code have?

• Defective alternator or generator
• Damaged wiring in the alternator control circuit
• Loose or corroded plug connections
• Defective engine control unit (ECM/PCM)
• Broken ground wire
• Faulty voltage regulator
• Defective alternator brushes
• Short circuit in the electrical system

What symptoms occur in parallel with fault code P0621?

• Battery problems: Weak or discharged battery despite driving
• Lighting faults: Fluctuating or dimmed headlights and interior lighting
• Starting problems: Difficulty starting the engine, especially in cold temperatures
• Electrical failures: Irregular operation of radio, air conditioning or other electrical consumers
• Voltage warning: Battery or charge indicator light in the dashboard lights up
• Loss of engine power: Reduced engine power due to insufficient power supply

How to fix the problem of error code P0621?

• The repair requires a systematic approach to identify the exact source of the fault.
• Complete alternator check and replacement: Test the alternator with a multimeter for proper voltage output (13.5-14.4V with the engine running) and replace it together with the voltage regulator if it is defective
• Wiring diagnosis and repair: Check all cables from the alternator to the control unit for continuity, insulation faults and corrosion, repair or replace damaged cables completely
• Clean and secure plug connections: Disconnect all relevant plugs, clean contacts with contact spray, check for tight fit and replace corroded plugs
• ECU diagnostics and reset: Carry out a deep diagnosis of the engine control unit, update the software if available or replace the ECM if there is evidence of a fault
• Check and replace ground connections: Locate all ground points of the charging system, clean them thoroughly and make tight connections, add additional ground wires if necessary