Error Code P0623 is defined as Generator Lamp Control Circuit, meaning there’s a voltage problem detected between the PCM and generator control module, usually caused by battery or alternator, weak, or dead battery.

This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, mainly those made since 1996 up to present. It’s particularly common among Chevrolet, Dodge, Ford, GMC, and Lincoln. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make, model, or powertrain configuration.

When Error Code P0623 is stored, that means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has determined a malfunction with the generator lamp control circuit.

The generator lamp is housed in the instrument panel, and its primary purpose to warn the driver of a potential problem in the charging system when it lights up.

The PCM monitors the continuity of the generator lamp control circuit every time the engine is running. The generator lamp control circuit plays a vital role in the operation and maintenance of battery charge levels.

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Every time the ignition is on and the PCM is energized, both the APP sensor and TPS self-tests will commence. Aside from running the internal controller self-test, the CAN (controller area network) also compares signals from each individual module to ensure that each controller is running correctly.

If the PCM detects a problem while monitoring the generator lamp control circuit, then it will register the Error Code P0623 and activate the Check Engine light. Depending on the severity of the condition, it may take multiple failure cycles before the Check Engine light lights up.

Issues with the Internal Control Module must always be taken seriously, as codes like this one cause drivability concerns such as no-start and dead battery condition.

Common Symptoms

Common symptoms for this code include:

  • Engine stalling when idling
  • Various drivability issues
  • Inadvertent engine shutoff
  • Delayed engine cranking

In some cases, it may come with other stored codes.

Possible Causes

There are many possible causes for this code, such as:

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  • Defective PCM or PCM programming error
  • Defective alternator/generator
  • Open or shorted generator lamp control circuit
  • Defective alternator/generator
  • Defective generator lamp bulb

How to Check

As with most error codes, diagnosing this fault requires a diagnostic scanner, battery/alternator tester, DVOM (digital volt/ohmmeter), and a reliable vehicle information source.

Look for the vehicle information from the TSB (technical service bulletin) that is parallel with the code stored, vehicle (year, make, and model), engine, and symptoms. If you find the right TSB, then this will give you the right diagnostic information.

Start the diagnosis by connecting the scanner to the vehicle’s diagnostic port and retrieve all stored codes, as well as their freeze frame data. You may write this information down, in case the problem proves to be intermittent. After recording necessary data, clear the codes and then take the vehicle for a test drive until the code resets, or the PCM enters readiness mode. If the PCM enters Readiness Mode, that means the code is intermittent, and the problem must develop further before you can accurately diagnose and do the necessary repair.

Then, test the battery using battery/alternator and make sure it’s charged sufficiently. If it’s not, then test the alternator/generator. Make sure you follow the specifications and instructions set by the manufacturer and the maximum voltage output requirements for the alternator and battery. If the alternator/generator is not charging, then go to the next step.

Use vehicle information source to obtain component locations, connector face views, connector pin-out charts, wiring diagram, and diagnostic flow chart related to your vehicle’s error code.

Then, check to see if there’s a battery voltage on the alternator/generator warning lamp circuit. Use the appropriate wiring diagram and DVOM. If there’s none, check system fuses and relays and replace defective components as needed. If there’s a voltage in the generator warning lamp, then you can suspect a defective alternator/generator warning lamp bulb.

If the alternator is charging, that means the alternator/generator warning lamp bulb is functioning correctly, but if P0623 still resets despite that, then use a DVOM to test the controller power supply fuses and relays. Replace blown fuses as needed. Fuses must be tested with the circuit loaded.

If all fuses and relays are running well, check whether the controller related wiring and harness are in order. You may also want to check chassis and engine ground junctions. Again, use your vehicle information source to obtain ground locations as related circuits. Test ground integrity using the DVOM.

Then, check the system controllers for any signs of water, heat, or collision damage. Any controller that is damaged, especially by water, is considered defective and must be replaced.

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If controller power and ground circuits are intact, then there’s a good chance the controller is defective, or there’s a programming error in the controller. Thus, controller replacement requires reprogramming. In some cases, you may want to get aftermarket reprogrammed controllers. Some vehicles and controllers require on-board reprogramming that may be done only through the dealership or a qualified shop.

How to Fix

Depending on your diagnosis, common repairs for this code include:

  • Replacement of battery
  • Replacement of alternator
  • Replacement of damaged wiring
  • Replacement of battery cables and connectors

As with most codes related to the PCM control circuit, searching for the exact cause of this problem can be tricky. Thus, it’s important to start diagnosis with the easiest before thinking of replacing any parts like the alternator, battery, or PCM.

This code may cause the vehicle not to start, or worse, the battery will not charge while driving.