P0622
PowertrainGenerator Field F Control Circuit Malfunction
The ECU controls how hard the alternator charges by switching the field coil winding on and off through a control wire. When it commands the field and the voltage or current coming back doesn't match what it expects, it logs P0622. For you as the owner, that means the charging system isn't behaving properly, so the battery might not be getting topped up correctly while you drive.
ⓘ Information only. This page provides general educational information about fault code P0622. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0622 mean?
P0622 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Generator Field F Control Circuit Malfunction.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Battery warning light glowing on the dash, usually the first thing you'll notice
- • Headlights and dash lights dimming or pulsing, worst at idle and when you turn on a load like the heated rear screen
- • Battery going flat after a few days even though you've been driving it
- • Radio, electric windows or climate control cutting in and out
- • Hard starting or the engine cutting out after a longer drive once the battery has drained
- • Charging warning sometimes coming and going, which points to a loose connection rather than a dead alternator
Possible causes
Causes commonly associated with P0622, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Worn alternator or its internal voltage regulator. On a high-mileage car this is the usual culprit, as the regulator and brushes wear and lose proper control of the field
- 2. Corroded, chafed or broken wiring in the field-control circuit between the ECU and the alternator
- 3. Bad pins or a loose plug at the alternator's field terminal, often green with corrosion on older cars
- 4. A blown fuse or tired relay feeding the charging circuit
- 5. Poor earth at the engine or chassis, which throws off the voltages the ECU is reading
- 6. ECM/PCM fault or a software glitch, which is rare and worth ruling out everything else first
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Measure battery voltage with the engine off, then again at idle with the engine running. A healthy system sits around 13.5 to 14.5V. If it stays at battery voltage, around 12.5V or less, the alternator isn't charging
- 2. Read all the stored and pending codes, not just P0622. Fuel trim or other charging codes alongside it change the picture
- 3. Wiggle-test and inspect the field-terminal connector and harness for corrosion, green crust, melted insulation or a loose pin. This is where a lot of these faults hide
- 4. Check the charging-circuit fuses and any relay with a multimeter under load, and look for a short before you just replace a blown fuse
- 5. Clean and tighten the engine and chassis earth straps, then recheck your voltages
- 6. If the wiring and earths are sound, get the alternator load-tested to confirm the field coil is doing its job before you condemn anything
Common questions about P0622
If it's the alternator, is a cheap aftermarket one fine or should I pay for OEM? +
A good-quality remanufactured or branded aftermarket alternator from a reputable UK motor factor is fine for most everyday cars and saves you a fair bit over OEM. Brands like Valeo, Denso or Bosch reman units are the ones to look for. Avoid the cheapest no-name eBay alternators, because the regulators in them can be flaky and you may land back with the same P0622 a few months later. On newer cars with smart or variable-charging alternators, stick closer to OEM spec, as the cheap ones don't always talk to the ECU correctly.
Can I keep driving with this code showing? +
Short trips, maybe, but don't rely on it. If the alternator isn't charging properly the battery will slowly drain, and once it's low the lights dim, the electrics start dropping out and the engine can stall. You don't want that happening on a dark motorway. Get it looked at within a day or two rather than ignoring it for weeks.
Will P0622 fail my MOT? +
The code on its own isn't an MOT item, but the battery or charging warning light is checked. If that light is lit at the time of the test the car will fail on it. Sort the underlying fault, clear the light and make sure it stays off before you book the MOT.
What sort of money am I looking at to fix it? +
It depends what's actually wrong. A corroded connector clean or a blown fuse might cost very little. Wiring repairs tend to run into the low to mid hundreds once you factor in diagnosis time. A replacement alternator at a good independent garage is often somewhere around £200 to £450 fitted depending on the car, while a main dealer or anything needing module work can push toward four figures. Get the diagnosis done first so you're not paying for an alternator when it was a £15 plug.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →