OBD2 codes.

Generator Field/F Terminal Circuit

The alternator recharges the battery while the engine runs, and the engine computer controls how much it charges through control wires. This code means a fault in the F circuit, through which the engine computer receives how hard the alternator is working (the field signal).

What the car has detected

The F circuit signal doesn't make sense: with the ignition on and engine off it should be near zero, and with the engine running it should vary within a range, and it doesn't.

If the battery light comes on with the engine running, the alternator may not be charging: the car will run on the battery until it is flat and then stop. Go to a garage soon and avoid switching on unnecessary loads.

Symptoms

• Battery light on with the engine running
• Engine warning light on
• Battery running flat
• Lights or accessories failing if the battery gets very low

Possible causes

• F control wire broken, poorly connected or shorted
• Loose or corroded alternator connector
• Failed alternator or voltage regulator
• Control fault in the engine computer (less common)

Checks for the garage

• Check battery voltage and charging voltage with the engine running
• Check the alternator's main supply (B+) and earth
• Check the F control wire between the alternator and the engine computer for breaks or shorts
• Look at the alternator control signal with diagnostics, engine off and running
• Replace the alternator or engine computer only after checking the wiring

Next step

Have the charging voltage, the alternator control wire and its connector checked.

Go to a garage soon, especially if the battery light is on. Charging circuit checks need a multimeter and diagnostics.

Original sources · Date indicated: 2026-10-12