P0621 is a generic OBD2 code, which means that it has the same meaning for the Honda Legend as it would any other vehicle. However, it can be challenging to diagnose due to the necessity of removing the instrument cluster.
OBD2 P0621 indicates an issue with the circuit that controls the generator lamp, which is commonly referred to as the alternator/battery light. The most common cause of this code is a bad bulb in the instrument cluster.
This article will provide you with the definition of P0621 and some practical steps that you can take at home to determine the cause of this code in your Legend.
P0621 Definition: Generator Lamp “L” Control Circuit Malfunction
Here’s the definition of P0621, broken down by its two halves.
Generator Lamp “L”
The generator lamp is in your Legend’s instrument cluster and warns you when the charging system has a problem. The “L” means that the lamp control circuit is what we are dealing with and is redundant.
Control Circuit Malfunction
The circuit that controls the generator lamp has failed to complete its readiness check when the ignition is turned on. There are several potential causes.
Sometimes it can take several engine on/off cycles for P0621 to appear in the Honda Legend’s PCM (powertrain control module) memory.
P0621 Symptoms: Honda Legend
Here are the common symptoms of P0621 in the Legend. The check engine light may be that the only thing your vehicle is experiencing. The symptoms below would be indicative of a charging issue:
- Stalling engine
- Engine dies while driving and won’t start back up
- Slow start
- Check light (with codes stored other than P0621)
If there are codes other than P0621, and they’re charging system-related, cross-reference them with the causes listed below. It can narrow your search for a potential cause.
Honda Legend P0621 Causes + Diagnosis
The most likely causes of P0621 in the Honda Legend are related to the wiring harness going to the generator lamp bulb or the generator lamp bulb itself failed.
Other less likely causes are a bad alternator or a PCM-related issue. Here is an excellent order to go about diagnosing P0621.
1. Check for Other Codes
The best place to start is checking for other codes. If there are different codes related to the charging system, it can provide valuable insight into what is causing P0621.
2. Is the bulb turning on?
Whenever you put the key in your Legend’s ignition and turn it to the run position, the alternator/battery light should stay on for a few seconds and then turn off. If it doesn’t turn on at all, the bulb itself is likely burnt out.
If your Legend’s generator light is stuck on and won’t turn off, jump to step 6.
3. Check What You Can BEFORE Removing the Instrument Cluster
Are any of the other lights not working on the instrument cluster? They should all turn on for a moment when the key is placed in the run position (except the turn signals).
Some makes and models have fuses for the instrument cluster lights. If there is one, verify it’s not blown. If it is, replace it. If it goes out again, you have a wiring problem between it and your vehicle’s PCM.
If a few of them aren’t working, it indicates there’s likely an issue with the wiring in the instrument cluster itself. If a few lights appear bad, or it appears only a certain portion of the instrument cluster is working, it’s likely what’s causing P0621 in your Legend.
4. Check and Replace the Bulb
At this point, assuming the generator light is not coming on, but everything else on the cluster is working fine, it’s time to check to see if the bulb is bad.
Get a new bulb and throw it in there. That’ll more than likely be the fix for P0621 at this point. If it’s not, it’s time to check the wiring harness.
5. Check the Wiring (Bulb Not Illuminating)
With the harness disconnected from the instrument panel, check if the circuit is hot where it would plug into the instrument panel. If it is, but the bulb was off (and you’ve replaced it), there’s an issue with the instrument cluster.
If it’s not hot at the cluster harness, you’ve got a wiring problem somewhere between your Legend’s PCM and the harness.
6. Check the Wiring (Alternator Light Stuck On)
If the harness is hot at the instrument cluster with the alternator circuit commanded off, then you know there’s a short somewhere between the battery light and PCM.
With the instrument panel disconnected, check the alternator light pin to see if it’s hot. If it is, you know the problem is the harness under the dash. If it is not hot, it’s likely an instrument cluster issue.
On the rarest of occasions, the PCM can be the cause of failure with P0621. It may need to be reprogramed or replaced.
8. Bad Alternator or Alternator Controller
You’ll need to test the alternator controller to see if it is functioning properly. Check so make sure that it is grounded well (usually not a problem). You’ll need to get a wiring schematic for your Legend’s model year and engine to make sure you’re looking at the right thing.
P0621 is not that hard to diagnose in the Honda Legend. It’s removing the instrument cluster that is challenging (if necessary). It’s more often than not caused by a failed generator lamp bulb.