2000 Cadillac STS P0621 Generator L Terminal

[STSMAN76]

Hi everyone,

I have a 00 Seville STS and last week the Battery Light and the Service charging system/battery not charging light came on. The battery turned out to shot so I replaced it. I picked up a used alternator off of a 2000 Seville but when I brought it to my mechanic he called and said the the Voltage from the original alternator is good at 14.4V and that the battery is indeed charging. He didnt want to replace it if it didnt need to be replaced. The DTC P0621 is showing and I was wondering if anyone here has had this issue and knows what it is and how to fix it.

Thanks

Shayne

[Dave in Texas]

Hi everyone,

I have a 00 Seville STS and last week the Battery Light and the Service charging system/battery not charging light came on. The battery turned out to shot so I replaced it. I picked up a used alternator off of a 2000 Seville but when I brought it to my mechanic he called and said the the Voltage from the original alternator is good at 14.4V and that the battery is indeed charging. He didnt want to replace it if it didnt need to be replaced. The DTC P0621 is showing and I was wondering if anyone here has had this issue and knows what it is and how to fix it.

Thanks

Shayne

Could be the system 'seeing' high-turn On voltage. Can also be caused by wrong spark plugs being used.

[BodybyFisher]

P0621

The PCM uses the L-terminal circuit to control the load of the generator on the engine. A high side driver in the PCM applies a voltage to the voltage regulator. This signals the voltage regulator to turn the field circuit ON and OFF. When the PCM turns ON the high side driver, the voltage regulator turns ON the field circuit. When the PCM turns OFF the high side driver, the voltage regulator turns OFF the field circuit.

The PCM monitors the state of the L-terminal circuit. The PCM should detect a low L-terminal voltage when the key is ON and the engine is OFF, or when the charging system malfunctions. With the engine running, the PCM should detect a high L-terminal voltage. The PCM performs key ON and RUN tests to determine the status of the L-terminal circuit. During the key ON test, if the PCM detects a high L-terminal voltage, DTC P0621 will set. DTC P0621 will also set if, during the RUN test, the PCM detects a low L-terminal voltage. When the DTC sets, the PCM will send a class 2 serial data message to illuminate the charging telltale

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If that alternator that you have installed is an aftermarket alternator, we have seen those cause this DTC code. We only recommend ACDELCO alternators that are NEW or REBUILT.