GMC's mid-size SUV -- the Envoy -- comes equipped with the Passlock anti-theft system. A resistor in the device sends a signal allowing the vehicle to start when a properly coded key is inserted in the ignition. Some owners have experienced problems with the system; the vehicle might not start and the anti-theft light on the dashboard might light up. The system must be reprogrammed if this situation occurs.
Insert the key in the ignition, and turn it to the "on" position. This causes the anti-theft light to blink on and off. Leave the key in the ignition for 10 minutes.
Turn the key to the "off" position -- and leave it there for 20 seconds -- after the anti-theft light stops blinking. This disables the Passlock system; it is ready to "relearn" your key.
Remove the key. Put the key back in the ignition, and start the vehicle.
