P0622 — Generator Field Control Circuit Malfunction

What P0622 means

The engine control module regulates the alternator's field current to keep system voltage in range. P0622 sets when the signal in the field control circuit doesn't match what the module expects, such as an open, a short to ground or power, or a failed regulator. The alternator may then undercharge, overcharge or not charge at all.

Symptoms

• Battery warning light or check engine light on
• Battery drains or won't charge
• System voltage below 13 V or above 15 V with engine running
• Dim headlights or weak electrical accessories
• Hard starting after sitting
• Unstable idle

Possible causes

1. Failed voltage regulator (usually built into the alternator)
2. Open or shorted wiring to the alternator field terminal
3. Corroded or loose alternator connector
4. Worn brushes or a faulty rotor in the alternator
5. Weak or aging battery
6. Rarely, a faulty engine control module

How to diagnose

1. Scan for codes and freeze-frame data, and look for other charging or voltage codes
2. Measure battery voltage with the engine off and running (about 13.5-14.7 V is normal)
3. Inspect the alternator connector and harness for corrosion, heat damage or chafing
4. Check the drive belt condition and tension, plus battery terminals and ground straps
5. Test the control wire for continuity from the module to the alternator and for shorts
6. Check power and ground at the alternator connector per the wiring diagram
7. Check the control signal with a scan tool or oscilloscope
8. Load-test the battery and bench-test the alternator

Common fixes

• Clean and repair the alternator connector, or replace it
• Repair the damaged wire or harness section
• Replace the regulator/brush assembly or the whole alternator
• Replace a weak battery
• Clear the code and verify charging; replace the module only after everything else is ruled out

Questions and answers

What does P0622 mean?

It's a fault in the alternator field control circuit. The engine computer sees an incorrect signal there and can't regulate charging properly.

Can I drive with P0622?

Only a short distance to a repair shop. If the alternator isn't charging, the battery will run down quickly and the engine can stall.

Is P0622 always a bad alternator?

No. Wiring, a corroded connector or a weak battery can also cause it, so test the circuit and charging voltage before replacing the alternator.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.