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P0633 Immobilizer Key Not Programmed – ECM/PCM

Check Engine Light

P0633 code definition

The P0633 code is triggered when the vehicle's PCM (Powertrain Control Module) or ECM (Engine Control Module) is unable to recognize the immobilizer key.

What the P0633 code means

P0633 is cued when there is an internal malfunction or improper programming resulting in the PCM being unable to recognize the immobilizer key. The immobilizer is an engine component that prevents it from running without the proper electronic immobilizer key. The code indicates that the vehicle will not start because the proper key is not in use.

What causes the P0633 code?

This code is most commonly caused by the following:

  • The PCM has been recently replaced but did not receive the programming required to match the vehicle and/or immobilizer key.
  • A fault in the PCM.
  • A defective ignition cylinder.
  • Open or shorted connectors or wiring.
  • A defective immobilizer key.
  • A dead battery that is discharged for a lengthy period of time can cause the PCM to lose its programming, and the need for a reprogramming can be the cause of this code.

What are the symptoms of the P0633 code?

P0633 code typically cues the vehicle to illuminate a security lamp or key indicator that signals a "no start" will occur.

How does a mechanic diagnose the P0633 code?

The P0633 is diagnosed with an OBD-II scanner, but the use of a digital volt/ohmmeter is also important in properly diagnosing this issue. The standard protocol begins with a visual inspection of the vehicle's wiring. Any problematic issues must be addressed, including replacing or repairing shorted, disconnected, damaged or corroded wires and connections. Components and fuses in need of repair or replacement are addressed at this time too.

The system is then re-tested and if the wiring is functioning properly, the mechanic may then connect the OBD-II scanner to the vehicle's diagnostic connection and document the codes and freeze frame data it contains. It is important to note any data or irregular issues that may have played a role in the code storage. Clear the code and attempt to operate the vehicle as this can indicate if that is the source of the problem. If it is not an electrical or wiring issue, it is likely that the PCM will need diagnosis and/or replacement. Typically this is more complex and requires the use of an Autohex or specialized scanner.

A mechanic must also assess any recent repairs, as any changes in a control module may lead to a reprogramming-related problem, and the P0633 code. It may also be necessary to use an auxiliary immobilizer key to test the current key's authority. If none of these yield results, the final test to perform is on the ignition cylinders as some vehicle types trigger this code when a cylinder is defective.

Common mistakes when diagnosing the P0633 code

This is a very rare code with a very narrow number of causes. There are rarely misdiagnosis or mistakes to be made when this code is triggered.

How serious is the P0633 code?

This is a very serious issue because it prevents the vehicle from starting. Though helpful in the prevention of "hot wiring" and auto theft, when it is a malfunction in the immobilizer key or the PCM/ECM it locks down the engine and prevents the vehicle from operating at all. Unfortunately, diagnosis and resetting the codes is not always going to address the problem because reprogramming may be required. The vehicle ID, mileage, and any anti-theft settings may need to be reset before the vehicle can be operated.

What repairs can fix the P0633 code?

The most common repairs required for the P0633 code are:

  • Verifying with code reader or scanner, and clearing the code to see if the issue returns.
  • Determining if there was intermittent issue that triggered the code, such as an electrical issue. This is why diagnosis also requires visually inspecting all of the connections and repairing or replacing wiring, connections, components and fuses before retesting.
  • A PCM or another control module, ECM for example, may need to be replaced and then reprogrammed to match the vehicle.
  • The use of an auxiliary immobilizer key may be necessary to gauge the key's authenticity and to reprogram any new control modules properly.
  • Repairing this issue can also mean assessing and repairing defective cylinders in the ignition.

Additional comments for consideration regarding the P0633 code

Never replace a PCM with a used component as this will not alleviate this issue. The OBD systems can function exclusively in the one vehicle, programming everything into the PCM or ECM, and then relaying and sharing information with other control modules through the CAN bus network. This code can mean a new PCM/ECM and that means reprogramming the unit to match the vehicle.

Need help with a P0633 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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