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Code P0621: Meaning, causes and diagnosis


P0621 is a generic OBD2 code, which means it has the same meaning for any vehicle manufactured since 1996. Although it is a generic code, it occurs most frequently on vehicles manufactured by GM and Fiat.

The OBD2 code P0621 indicates that there is a problem with the circuit that controls the generator light, commonly known as the alternator/battery light.

This article will provide you with the definition of P0621, as well as some practical steps you can take at home to diagnose this code.

P0621 Definition: Generator Lamp Control Circuit Malfunction “L”

Here is the definition of P0621, broken down into its two halves

Generator lamp “L”

P0621 Definition
The typical generator light looks like a battery. Some vehicles have an alternator light and a battery light.

The generator light is on the instrument panel and alerts you when the charging system has a problem. The “L” just means that it is the lamp control circuit, and is redundant.

Control circuit malfunction

The circuit that controls the generator lamp has not completed its readiness check when the ignition is turned on. There are several reasons that can cause this. In the causes + diagnosis section below we will see in what order they should be addressed.

Sometimes it may take several engine on-and-off cycles for P0621 to appear in the PCM (Powertrain Control Module) memory.

P0621 Symptoms

P0621 Symptoms

These are the common symptoms of P0621. The check engine light may be the only thing your vehicle experiences. The following symptoms would be indicative of a charging problem:

  • The engine stalls
  • The engine shuts off while driving and won’t start again
  • Slow start
  • Ignition failure
  • Control light (with stored codes other than P0621)

If there are codes other than P0621, and they are related to the charging system, cross-reference them with the causes below, it can greatly narrow your search.

P0621 Causes + Diagnosis

The most likely causes of P0621 will be related to the wiring harness that goes to the generator light bulb, or to the failure of the generator light bulb itself.

Other less likely causes are a bad alternator or a PCM-related problem. Here is a good order to diagnose P0621.

1. Check other codes

The best place to start is to check for other codes. If there are other codes related to the charging system, it may provide valuable information about the cause of P0621.

2. Does the light bulb turn on?

Whenever you put the key in the ignition and turn it to the run position, the alternator/battery light should stay on for a few seconds and then go off. If it doesn’t light up at all, it’s very likely that the bulb itself is burned out.

If your vehicle’s generator light is on and won’t go off, go to step 6.

3. Check what you can BEFORE removing the instrument panel

Are none of the other lights on the instrument panel working? They should all come on momentarily when the key is turned to the run position (except the turn signals).

Some makes and models have fuses for the instrument panel lights. If there is one, make sure it is not blown. If it is, replace it. If it melts again, you have a wiring problem between it and your vehicle’s PCM.

If some of them do not work, it indicates that there is probably a problem with the wiring of the instrument panel itself. If some of them seem wrong, or only part of the instrument cluster seems to be working, it is likely the cause of P0621.

4. Check and replace the bulb

At this point, assuming the generator light doesn’t come on but everything else on the dashboard is working fine, it’s time to check for a bad bulb.

Get a new bulb and put it in. That’s most likely the solution for P0621 right now. If it isn’t, it’s time to check the wiring harness.

5. Check the wiring (the bulb does not light up)

With the harness disconnected from the instrument panel, check to see if the circuit is hot where it plugs into the instrument panel. If it is, but the bulb was out (and you replaced it), there is a problem with the instrument panel.

If it is not hot in the instrument cluster harness, you have a wiring problem somewhere between the PCM and the harness.

6. Check the wiring (alternator light on)

If the wiring harness is hot in the instrument cluster with the alternator circuit disconnected, then you know there is a short somewhere between the MIL and the PCM.

With the instrument cluster disconnected, check the alternator light plug to see if it is hot. If it is, you know the problem is the harness under the dashboard. If it is not hot, it is likely an instrument cluster problem.

7. PCM

In rare cases, the PCM may be the cause of the fault with P0621. It may need to be reprogrammed or replaced.

8. Defective alternator or alternator controller

You will need to check the alternator controller to see if it is working properly. Check that it is well grounded (normally not a problem). You will need to get a wiring diagram for your year and model of vehicle to make sure you are looking at the right thing.


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