Does P0620 mean I need a new alternator?

Not necessarily. P0620 describes a fault in the circuit through which the engine control unit commands the alternator, and that circuit includes a connector, a length of wire and, on many cars, a battery monitoring sensor on the negative terminal. Any of those can fail with a perfectly good alternator. That said, the regulator module inside the alternator is a common failure point and is usually not supplied separately, so the alternator does end up being replaced in a fair proportion of cases.

Why does the engine control unit control the alternator?

For fuel economy and battery management. Driving an alternator hard takes real power from the engine, so modern cars reduce charging under acceleration and increase it when coasting or braking, recovering energy that would otherwise be wasted. Doing that needs the engine control unit to command the alternator rather than letting it regulate itself. The side effect is a control circuit that can develop faults of its own, which is what P0620 reports.

Will P0620 fail an MOT?

Yes, on the lamp. An illuminated engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, and 371,859 failures have been recorded against it. The charging system itself is not part of the MOT, but a car that goes flat and cannot be kept running through the test cannot be tested, and an insecure or leaking battery is a defect a tester will record.

Why has my stop-start stopped working with P0620?

Because stop-start depends on the car being confident it can restart the engine, and that confidence comes from the battery management system. With a fault in the alternator control circuit the control unit no longer knows the battery's true state of charge, so it disables stop-start as a precaution. It is a symptom rather than a separate fault, and it usually returns on its own once the charging fault is repaired and the battery has recovered.

Powertrain code · standard

P0620Alternator control circuit fault

Standard definition: Generator Control Circuit

Last updated 27 July 2026

Quick answer

The engine control unit governs how hard the alternator charges, and it has found a fault in the circuit it uses to do that. The charging may be too low, too high or simply uncontrolled, and the battery warning lamp is usually on as well.

How the car works this out

On most cars built in the last fifteen years or so the alternator is not self-regulating. The engine control unit tells it what to do, over a dedicated wire or a low-speed data line, so that charging can be reduced under acceleration and increased when coasting. The unit monitors that control circuit and the alternator's reported status. P0620 is set when the circuit is open, shorted, or the alternator is not responding to the commands it is being given. It is a control fault rather than a statement about the resulting voltage, which is what separates it from P0562 and P0563.

What you might notice

Battery charge warning lamp on, sometimes flickering
Battery going flat, or the car struggling to start after standing
Headlights dimming or surging with engine speed
Other warning lamps appearing as system voltage wanders
Stop-start function no longer working
Occasionally no symptoms other than the engine lamp

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1A failed alternator, or a failed regulator module within itCommon
2A broken, chafed or corroded wire in the alternator control circuitCommon
How to check: Inspect the small control plug on the back of the alternator. It sits in a hot, dirty part of the engine bay and its terminals corrode.
3A faulty battery monitoring sensor on the battery negative terminal, so the control unit is commanding charging based on wrong informationPossible
How to check: Read the sensor's live values and compare its reported voltage and current against a direct measurement.
4A poor earth on the alternator or the engine-to-body strapPossible
5A battery replaced without being registered to the car on systems that require itPossible
6A fault within the engine control unit's alternator driver, which is uncommonLess common

What to do about it

1Check the small control connector on the alternator before condemning the unit. Corrosion or a broken wire there is far cheaper to put right than an alternator.
2Have the charging voltage measured across the battery terminals with the engine running, so you know whether the car is undercharging, overcharging or charging normally despite the code.
3Have the battery load tested at the same time. A tired battery and a control circuit fault often turn up together, and replacing one without the other leads to a repeat visit.
4Ask whether the battery monitoring sensor has been checked on cars fitted with one. It is a much smaller part than an alternator and produces very similar symptoms.
5Make sure any new battery is registered to the car where the system requires it.

Can you keep driving?

Only to a garage

If the alternator is not charging properly, the car is running on stored battery capacity and will eventually stop. Losing the engine takes the power steering assistance and the brake servo with it. Switch off the heated rear window, the heater fan and anything else you can spare, keep the journey short, and do not switch the engine off until you are somewhere it can be looked at.

Will it fail an MOT?

The engine management lamp is a major defect with 371,859 recorded failures against it. Beyond that, the MOT does not test the charging system, so the alternator's condition is not assessed directly — but a car that cannot be started or kept running for the duration of the test cannot be tested, and the tester will record a battery that is insecure or leaking. Bear in mind too that a flat or disconnected battery resets the readiness monitors, which can stop the emissions check being completed for some time afterwards.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Does P0620 mean I need a new alternator?: Not necessarily. P0620 describes a fault in the circuit through which the engine control unit commands the alternator, and that circuit includes a connector, a length of wire and, on many cars, a battery monitoring sensor on the negative terminal. Any of those can fail with a perfectly good alternator. That said, the regulator module inside the alternator is a common failure point and is usually not supplied separately, so the alternator does end up being replaced in a fair proportion of cases.
Why does the engine control unit control the alternator?: For fuel economy and battery management. Driving an alternator hard takes real power from the engine, so modern cars reduce charging under acceleration and increase it when coasting or braking, recovering energy that would otherwise be wasted. Doing that needs the engine control unit to command the alternator rather than letting it regulate itself. The side effect is a control circuit that can develop faults of its own, which is what P0620 reports.
Will P0620 fail an MOT?: Yes, on the lamp. An illuminated engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, and 371,859 failures have been recorded against it. The charging system itself is not part of the MOT, but a car that goes flat and cannot be kept running through the test cannot be tested, and an insecure or leaking battery is a defect a tester will record.
Why has my stop-start stopped working with P0620?: Because stop-start depends on the car being confident it can restart the engine, and that confidence comes from the battery management system. With a fault in the alternator control circuit the control unit no longer knows the battery's true state of charge, so it disables stop-start as a precaution. It is a symptom rather than a separate fault, and it usually returns on its own once the charging fault is repaired and the battery has recovered.

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