Error Code P0620 is defined as Generator Control Circuit Malfunction. This code means there’s an abnormal reading detected coming from the ECM, which is usually caused by a defective generator or faulty voltage regulator.

This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system, especially those made since 1996 up to present. It’s particularly common, but not limited to cars from Buick, Cadillac, Chevrolet, Ford, GMC, Mercedes Benz, and Hyundai. Specifications on the definition, troubleshooting, and repairs, of course, vary from make, model, and powertrain configuration.

When Error Code P0620 is stored, this means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) has detected a malfunction with the generator control circuit.

Every time the engine is running, the PCM energizes and monitors the generator control circuit.

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. These tests are simultaneously performed.

If there’s a problem detected in monitoring the generator control circuit, the Error Code P0620 will be stored, activating the Check Engine light altogether. Depending on the severity of the condition, it may take multiple failure cycles before the Check Engine light lights up.

Common Symptoms

This code shows a number of drivability issues, such as:

  • Engine drivability issues
  • Engine stalling when idling
  • Delayed engine cranking (especially if it’s cold)

Other codes may be stored.

Possible Causes

  • Defective PCM
  • PCM programming error
  • Open or shorted generator control circuit
  • Failed generator assembly
  • Insufficient control module ground

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 correct 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, use the DVOM to the test controller power supply fuses and relays. Next, test and replace any blown fuses as necessary. Fuses must be tested with 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.

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 faulty wires or other electronic components
  • Replacement or repair of generator
  • Repair or replacement of PCM (rare)

Unlike most codes, this code is likely caused by a defective controller or a programming error in the controller.

Test the integrity of the system ground by connecting the negative test lead of DVOM to the ground, and the positive test lead to battery voltage.