I see this a lot working on cars. The Passlock sensor inside the ignition switch goes bad intermittently and the 10 minute relearn procedure needs to be done in order to get the vehicle started back up. This procedure will need to be done anytime the Body Control Module loses it’s memory or anytime the Body Control module does not see the correct voltage from the ignition switch. For example, if the battery goes dead or gets replaced, if the BCM loses power for any other reason, (BCM fuse blows or is removed) if the wiring from the Passlock sensor to the BCM is bad or has a bad connection the relearn procedure may need to be performed. In severe cases, the BCM may even need to be replaced.
Note: This procedure will only work on GM vehicles that have this Passlock system
This Passlock sensor is located inside the ignition switch and receives a reference voltage (5 volts) from the Body Control Module when turning key on. This means that the sensor inside the ignition switch is considered to be a hall affect sensor. With the key on, it will then send a voltage reading back to the Body Control Module after it goes through the resistor. The voltage going back to the BCM will be considerably lower than 5 volts because it went through the resistor thus lowering the voltage. Overtime, the BCM will “learn” this reading. The voltage reading coming back into the BCM needs to be the same every time the key is turned on and to the crank position. If not, it will send a signal to the PCM preventing the the engine from starting by disabling the injectors and sometimes the fuel pump (depending on the year). Sometimes the BCM will “forget” this value or sometimes the resistor inside the ignition switch will go bad sending a different voltage reading back to the BCM when the key is turned on or to the crank position and will result in a no start condition. That is when a relearn procedure usually does the trick. If not, the ignition switch is bad and will need to be replaced. The wiring should also be checked before replacing the ignition switch. In extreme cases, the BCM may even need to be replaced.
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10 minute relearn procedure
Turn ON the ignition, with the engine OFF.
Attempt to start the engine, then release the key to ON, vehicle will not start.
Observe the SECURITY light. After approximately 10 minutes , the security light will turn OFF. The vehicle is now ready to relearn the Pass lock Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. Turn OFF the ignition, and wait 5 seconds. Start the engine, the vehicle has now learned the voltage reading coming back from the ignition switch.
Tip #1. When doing this procedure, the BCM is actually doing the learning. So some people might call it a “BCM 10 minute relearn procedure”.
Tip #2. If you want to do pinpoint testing using a voltmeter, you will need to get wiring diagram from ALLDATAdiy. They also have the removal and replacement procedure for the ignition switch if you need it.
In the video below, I had to do a 10 minute relearn procedure in order to get the vehicle started again after replacing the battery. (Sorry, my daughter was with me and she had a lot of questions)