Error Code P1631 is defined as Theft Deterrent Start Enable Signal Not Correct.

This error code is a manufacturer-specific trouble code, meaning it does not apply to all vehicles makes. Rather, it applies only to specific vehicle makes, such as Chevrolet, GMC, BMW, Chrysler/Dodge/Jeep, Citroen/Peugeot, Daewoo, Ford, Jaguar, Mazda, Opel, Saab, Saturn, Volkswagen/Audi, Volvo, etc. Specification on the definition, troubleshooting, and repairs still vary from one make and model to another.


The Class 2 serial data circuit is used for the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) to communicate with the VTD (Vehicle Theft Deterrent) BCM (Body Control Module) systems.

When the Passlock part of the VTD system has determined that the switch and lock have not been tampered and sensed proper operation of the ignition switch and lock, the BCM will transmit the password to the PCM. If the password matches to the one stored in module memory, fuel delivery will then commence. If any of the components of VTD system has been replaced, the module will need to relearn the password of the new components. If the relearn procedure was not performed, then the error code P1631 will set.

If this code happens during the ignition cycle that the PCM has enabled fuel, the BCM will enter into fail-safe mode: Fail Enabled VTD System Failure with Fuel Enabled. The BCM will remain in the fail-safe mode for present and future ignition cycles until the code is corrected and a valid password is received, or until the battery power is removed. If the codes are cleared or if the battery is disconnected, the vehicle will lose the Fail Safe mode status and will not start until the fault is solved, the 10-minute timer will expire, and the PCM will receive the right fuel delivery password.

Common Symptoms

As with other error codes, this code will activate the Check Engine light and register the code to the vehicle’s system. Also, since it’s a theft deterrent-related code, the vehicle will not start until the code is solved.

Possible Causes

There are a couple of possible causes for this error code to appear, such as:

  • The need to perform security relearn procedure
  • Defect in Ignition switch assembly
  • Open or shorted ignition switch harness
  • Poor electrical connection in ignition switch harness rubbed through wire insulation or broken wire inside the insulation

How to Check and Fix

You can conduct a security relearn procedure yourself by doing the following:

Insert key and try to start the vehicle, leave the key in On position. For some vehicles, both the battery and security lights may remain on. Leave the ignition on until the security lights go off. This may take about 5 minutes.

After the security light goes off, turn the ignition to the Off position, then leave it for 20 seconds.

Try to start the vehicle again and leave the key in the On position. For some vehicles, the battery and security light may blink for several seconds before shutting off. The security light will remain in On. Leave the ignition on until the security light goes off. This may take about 5 minutes. Then, turn the position into Off position for about 20 seconds.

Next, try to start the vehicle again, leave the key in On position. For some vehicles, both the battery and security light will blink for a few seconds before shutting off. The security light will remain on. Leave this on until the light goes. Again, wait for about 5 minutes.

After completing this procedure (3 times), your vehicle will start on your fourth attempt.

Note that some vehicles will have a different re-learn sequence. You may notice the security light, battery light, and abs light will alternate. After the security light is left out of sequence, that’s considered 1 step. Repeat the procedure two (or even more) times. The vehicle usually starts on the fourth try.

Also, it is important that you thoroughly inspect the circuitry which may be possibly causing the intermittent problem. Use the Testing for Intermittent and Poor Connections in the Diagnostics Aid of your vehicle. And if necessary, refer to Wiring Repairs or Connector Repairs in Diagnostic Aids.