Error Code P0648 is defined as Immobilizer Lamp Control Circuit, meaning there’s been an abnormal reading detected in the immobilizer lamp control circuit, which is likely caused by defective bulb or circuit, or faulty wiring.

This is a generic trouble code, meaning it applies to all vehicles equipped with the OBD-II system. This includes vehicle models from Acura, Buick, Chevrolet, Chrysler, Dodge, Ford, GMC, Honda, Hyundai, Kia, etc. Specifications on the definition, troubleshooting, and repairs, of course, vary from make, model, and powertrain configuration.

Error Code P0648 means the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes), has detected a malfunction in the immobilizer lamp control circuit.

Contained in the instrument panel, the job of the immobilizer lamp lights up to warn drivers should the immobilizer key malfunctions or if it doesn’t recognize the key. This warning lamp is also referred to as Security lamp for some vehicles. The PCM monitors the continuity of the lamp control circuit whenever the ignition is turned on. This immobilizer key comes with a transponder recognized by the PCM (and other controllers) when it’s in close proximity to the vehicle, allowing various systems (i.e., keyless entry, push-to-start) to remain functional and practical.

Whenever the ignition is turned on, and the PCM is energized, various controller self-tests are simultaneously performed. Aside from running internal controller self-test, the CAN (controller area network) carries serial data from each module to make sure the onboard controllers are interfacing properly.

When a problem in monitoring the immobilizer lamp control circuit occurs, then this code is stored and activates the Check Engine light.

Common Symptoms

  • Inoperative immobilizer lamp
  • No start condition

In many cases, this code also comes with other security codes.

Possible Causes

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

  • Defective or programming error in PCM
  • Open or shorted immobilizer lamp control circuit
  • Failed immobilizer lamp bulb
  • Defective (dead battery) immobilizer key (fob)

How to Check

Checking this code requires a DVOM (digital volt ohmmeter), dependable vehicle information for TSB (technical service bulletin) that has the code stored, vehicle (year, make, model, engine), and exhibited symptoms. Finding the right TSB can lead you to an easier and more successful diagnostic and repair procedure.

Start by connecting the scanner to the vehicle diagnostic port. Retrieve all the stored data including freeze frame data. Write down this information. Then, clear the codes and then take the vehicle for a test drive until the code reset, or the PCM enter readiness mode.

If the PCM enters readiness mode, that means the problem is intermittent. You may have to wait for the problem to get worse before you can successfully diagnose and repair. If the code resets on the other hand, then continue with the diagnosis.

Use the vehicle information to know the connector pin-out charts, face views, component locators, wiring diagrams, and diagnostic flow charts of the vehicle.

Use the diagram and DVOM to check the voltage of the battery on the speed control warning lamp circuit. If there’s none, check the system fuses and relays. Replace the defective parts as necessary.

If there is a voltage at the speed control warning lamp, then you can suspect a defective speed control warning lamp bulb.

If the speed control warning lamp bulb is working properly, and this code still resets, then use the DVOM to test the controller power supply fuses and relays. Replace as needed; fuses must be tested with the circuit loaded.

If all fuses and relays look good, then inspect the controller related wiring and harness. Check the chassis and engine ground junctions as well. Use your vehicle information source to locate ground for related circuits.

Also, check the system controllers; look for signs of damage, water intrusion, heat, or collision damage. Damaged controllers, especially those damaged by water, is considered defective and must be replaced.

If the controller power and ground circuits are in good shape, then you can suspect a defective controller or a programming error. In which case, controller replacement will be required, as well as reprogramming.

In some cases, you may want to purchase a pre-programmed controller from aftermarket sources. Other vehicles/controllers require on-board reprogramming that may be done through a dealership, or by a qualified technician.

How to Fix

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

  • Replacement of faulty components, connectors, and wirings
  • Replacement of defective CAN bus harness or the whole harness
  • Repair or replacement of straps, wires, and ground cables that are not properly connected
  • Replacement of faulty control modules (including PCM)
  • Replacement or reprogramming of PCM

One of the most common mistakes in addressing this code is treating any codes stored in reaction to the communication failure between the immobilizer lamp circuit and the PCM (or other control modules). In which case, this only treats the symptom and NOT the problem, causing the trouble code to return.