Fix P0621: Mechanic’s solutions that WORKED

The P0621: Generator Lamp “L” Control Circuit Malfunction points out a fault in the vehicle’s generator lamp “L” control circuit, which is part of the charging system. This circuit plays a crucial role in monitoring and controlling the alternator’s function. Problems in this circuit can lead to issues with charging the battery and powering the vehicle’s electrical systems.

P0621 quick overview

Meaning	P0621: Generator Lamp “L” Control Circuit Malfunction
Is it serious?	This code can be moderately serious as it indicates a potential charging system issue, which might affect the vehicle’s electrical systems and battery charging.
Possible causes	– Faulty or failing alternator – Damaged or corroded alternator wiring – Problems with the battery or its connections – Issues with the vehicle’s Powertrain Control Module (PCM) – Security system interference, especially after repairs or battery disconnection
How to diagnose?	Check Alternator Output: Verify the alternator’s voltage and current output, especially under load conditions. Inspect Wiring: Examine all wiring and connectors related to the alternator for any signs of damage or corrosion. Evaluate Battery Health: Ensure the battery is in good condition and the connections are secure and free of corrosion. Scan for Related Codes: Use a diagnostic scanner to check for any other related trouble codes that might provide further insight. Reset Security System: If the vehicle has a security system, perform a reset or relearn procedure if there have been recent power disruptions.

How to fix P0621 – learn from mechanics

I looked into P0621 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Chevrolet

Vehicle Information:
2000 Chevrolet Cavalier 2.2L

Issue + Repair History:

• Initially had a no-start problem.
• Fuel pressure was low; replaced the fuel regulator, and car ran fine.
• Car ran about 12 miles, then quit again.
• Replaced the fuel pump; car ran fine for a full tank of gas, then died.
• After being towed, car started and ran fine, but “trac off” light was on.
• Only memory code was P0621.
• Alternator output was low (12.5 volts at idle with A/C and headlamps on); replaced alternator with same results.
• Questioned if the issue was related to PCM.

Other OBD2 Codes + Test Results:
P0621 – Generator Term L Circuit

Suggestions to Fix Issue:

• Check crank and cam position sensor.
• Inspect the ground for the car; it’s located on the battery tray.
• Look into GM bulletin regarding battery terminal issues.
• Consider replacing the fuel filter as a clogged filter can damage the fuel pump.

Final Fix:

• The issue was resolved after the new fuel pump failed.
• Replaced the fuel pump again, and there were no further issues reported by the customer.

Case #2: Pontiac

Vehicle Information:
2001 Pontiac Sunfire SE 2.4L

Issue + Repair History:

• SES light on with intermittent “BATT” light.
• Codes P0622 and P0621 (Generator “F” and “L” control circuit issues).
• Initially had O2 sensor issues, which were fixed.
• Customer replaced the alternator but returned with the same codes.
• Wiring at the Gen F & L connectors checked; continuity between generator and PCM was fine.
• Alternator output at 14.3 volts to battery.

Other OBD2 Codes + Test Results:
P0622, P0621 – Generator “F” and “L” Control Circuit Problems

Suggestions to Fix Issue:

• Verify proper alternator model (AC Delco recommended).
• Check “F” terminal for being grounded at key on and ungrounded while running.
• Confirm “L” wire should have 5 volts from PCM to generator.
• Check for 0 volts at “F” terminal with key off and 12 volts with key on engine off.

Final Fix:

• Replaced with an AC Delco alternator, which resolved the issue.

Case #3: Saturn

Vehicle Information:
2004 Saturn Vue 3.5L

Issue + Repair History:

• Vehicle arrived with a dead battery and worn serpentine belt.
• Found alternator pulley seized (vehicle had been sitting for a while).
• Replaced alternator; charging rate at 12.4 volts and declining.
• Installed a second alternator with the same results.
• Inspected main battery wire from battery to starter and then to alternator; nothing found.
• Code P0621 stored in computer; cleared but returned after first startup.

Other OBD2 Codes + Test Results:
P0621 – Stored in computer

Suggestions to Fix Issue:

• Check if ECM is commanding the alternator.
• Inspect wiring for any stretching or damage.
• Look for a fuse for alternator or gauges.
• Test alternator with an incandescent test lamp.
• Consider the correct type of alternator for the vehicle.
• Check for a/c ripple on output side of alternator to battery.
• Examine engine earths, battery grounds, and alternator connector terminals.
• Verify battery voltage and amp output.
• Consider if the battery can hold a charge or pass a load test.
• Test the “Turn On” circuit voltage and check for opens, shorts, or ECM faults.
• Try a Delco alternator if aftermarket ones are problematic.
• Evaluate connections at the starter and fusible link or fuse.

Final Fix:

• After installing two remanufactured alternators without success and an OEM alternator being days away, the original alternator was rebuilt, which resolved the issue.

Case #4: Oldsmobile

Vehicle Information:
2001 Oldsmobile Alero GL 2.4L

Issue + Repair History:

• Car towed in with a no-start condition.
• Owner replaced the fuel filter.
• Confirmed good fuel pressure and good spark.
• No pulse to injectors.
• Alternator appears new, but no specific comments from customer about its replacement.
• Codes P0621 and P1632 present.
• No check engine light.
• Related incident of a no-start after alternator replacement at another shop, which was referred to a dealer (outcome unknown).

Other OBD2 Codes + Test Results:
P0621 – Malfunction generator control circuit
P1632 – Security fuel shut down

Suggestions to Fix Issue:

• Security system may need to be reset due to power disturbance (like changing the alternator).
• Perform relearn procedure as mentioned in the manual:
  1. Turn ignition to RUN.
  2. Attempt to start, then release ignition switch to RUN position.
  3. Observe security light; after about 10 minutes, light will go out.
  4. Turn ignition to OFF, wait 5 seconds.
  5. Repeat steps 1-4 two more times for a total of three cycles.
  6. Start the engine; the vehicle is now ready to relearn passwords.
  7. Use a scan tool to clear diagnostic trouble codes (DTCs).

Final Fix:

• The issue was resolved by resetting the security system as suggested. The vehicle required about 5 minutes of driving post-reset to function normally.

Case #5: Cadillac

Vehicle Information:
2002 Cadillac Seville SLS 4.6L

Issue + Repair History:

• Battery lamp was on.
• Codes P0621 and P0563 stored; reset when cleared.
• Charging voltage measured at 14.5 volts at battery and ignition one terminals.
• Tech 2 showed L terminal always off.
• Suspected short in alternator or possible PCM issue.
• Battery confirmed in good condition.

Other OBD2 Codes + Test Results:
P0621 – Malfunction in generator control circuit
P0563 – System voltage high

Suggestions to Fix Issue:

• Confirm if alternator can put out at least 5 amps more than the vehicle’s draw with all options turned on (AC, headlights, wipers, rear defogger).
• Consider the possibility of the alternator being the problem, as the regulator controls more than the charging circuits, especially in high-end cars.
• Check the condition of the belt and tensioner.
• Try a different alternator as a diagnostic step.

Final Fix:

• Replaced alternator with a new AC Delco unit.
• After replacement, the alternator had a constant battery voltage on the orange wire at the regulator, and the return wire showed a consistent wave pattern with no change regardless of load.
• The “L” terminal never received 5 volts before the fix.
• Cleared codes and tested the system under load with good results.

Advice	Diagnostic Step Description
Check Alternator Functionality	Verify the alternator’s ability to output adequate voltage and current, especially under load.
Inspect Wiring Connections	Examine wiring for any damage, stretching, or poor connections, especially at the alternator.
Consider Security System Impact	In vehicles with security systems, a reset or relearn procedure might be necessary.
Examine Battery Condition	Ensure the battery is in good condition, as a faulty battery can impact alternator performance.
Look for Related OBD2 Codes	Check for any related diagnostic trouble codes that might indicate a broader electrical issue.
Evaluate Alternator Control Circuitry	Assess the integrity of the alternator’s control circuitry, including the “L” and “F” terminals.
Try OEM or Specific Alternator Models	In cases of alternator replacement, consider using OEM or model-specific units for compatibility.
Perform Complete Charging System Check	Conduct a thorough examination of the entire charging system for any anomalies.