What does it mean?
P1668 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Generator L-Terminal Control Circuit .
Symptoms
- MIL on and the battery/charge lamp behaving oddly: never lighting at ignition on, or staying lit with the engine running
- Charging voltage low or missing, so the battery slowly drains
- Dim headlights, slow cranking and a flat battery after a few trips
- On some vehicles the alternator will not excite until the engine is revved
- Electrical accessories weak or cutting out under load
Likely causes
- Open in the L-terminal wire between the alternator and the module or cluster
- Corroded alternator plug, or a broken locking tab letting the small connector back out
- Failed voltage regulator inside the alternator
- Burnt charge warning lamp bulb on vehicles where the bulb carries excitation current
- Blown fuse in the L circuit
- Corroded pin at the module or cluster connector
- Wire chafed and shorted to ground or to battery positive
How to fix it
- Measure charging voltage at the battery with the engine running, first at idle and then under electrical load
- Check the charge warning lamp behaviour during the ignition-on bulb check; a dead bulb can stop excitation on some systems
- Inspect the small alternator connector: confirm it is fully seated and the pins are clean and not burnt
- Test continuity of the L wire between the alternator and the module or cluster, and check for shorts to ground and battery voltage
- Check the fuse feeding the L circuit and the connector pin at the module end
- Voltage-drop test the main charging cable and the engine ground straps
- Replace the alternator or its regulator only if the L signal is still absent with the wiring, bulb and fuse proven good