P0621 Generator Lamp L Control Circuit Malfunction
OBD-II Trouble Code Technical Description
Generator Lamp L Control Circuit Malfunction
What does that mean?
This is a generic diagnostic trouble code (DTC) and applies to many OBD-II vehicles (1996-newer). That may include but is not limited to vehicles from Chevy, Saturn, GMC, Opel, Porsche, Pontiac, etc. Although generic, the exact repair steps may vary depending on year, make, model and powertrain configuration.
A stored code P0621 means that the powertrain control module (PCM) has detected a malfunction with the generator lamp control circuit. The L simply reiterates the lamp control circuit.
The generator lamp is contained in the instrument panel. Its primary purpose is to warn the driver of potential charging system problems, when it is illuminated.
The PCM typically monitors the continuity of the generator lamp control circuit whenever the engine is running. The generator lamp control circuit is integral to generator operation and maintaining battery charge levels.
Each time the ignition is turned on and the PCM is energized, multiple controller self-tests are performed. In addition to running internal controller self-tests, the controller area network (CAN) is used to compare signals from each individual module to ensure that the various controllers are interfacing properly.
If a problem is detected in monitoring the generator lamp control circuit, a code P0621 will be stored and a malfunction indicator lamp (MIL) may be illuminated,. Depending upon the perceived severity of the malfunction, multiple failure cycles may be necessary for MIL illumination.
A typical XYZ:
What is the severity of this DTC?
Internal control module codes must always be taken seriously. A stored code P0621 could result in a variety of drivability concerns, including a no-start and/or dead battery condition.
What are some of the symptoms of the code?
Symptoms of a P0621 trouble code may include:
Engine drivability issues Engine stall when idling Inadvertent engine shutoff Delayed engine cranking Other stored codesWhat are some of the common causes of the code?
Causes for this code may include:
Faulty PCM PCM programming error Open or shorted generator lamp control circuit Defective alternator/generator Failed generator lamp bulbWhat are some P0621 troubleshooting steps?
A diagnostic scanner, a battery/alternator tester, a digital volt/ohmmeter (DVOM), and a source of reliable vehicle information will be required to diagnose a code P0621.
Consult your vehicle information source for technical service bulletins (TSB) that replicate the code stored, vehicle (year, make, model, and engine), and symptoms exhibited. If you find the appropriate TSB, it may yield helpful diagnostic.
Begin by connecting the scanner to the vehicle diagnostic port and retrieving all stored codes and freeze frame data. You will want to write this information down, just in case the code proves to be an intermittent one. After recording all pertinent information, clear the codes and test drive the vehicle until the code is reset or the PCM enters readiness mode. If the PCM enters readiness mode, the code is intermittent and will be more difficult to diagnose. The condition, which caused the P0621 to be stored, may even need to worsen before a diagnosis can be made. If the code is reset, continue with your diagnosis.
Use the battery/alternator tester to test the battery and make sure that it is sufficiently charged. If it is not, charge the battery as recommended and test the alternator/generator. Follow manufacturer’s recommended specifications for minimum and maximum voltage output requirements for the battery and alternator. If the alternator/generator is not charging, proceed to the next step in your diagnosis.
Use your source o
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