GM security systems come in three basic varieties depending upon the model and model year. Passkey 1 and 2 (1985 to 2003) use a resistor in the key that the computer must recognise in order to disable the security system. Passlock (1998 to 2003) uses a magnetic Hall Effect sensor, and Passkey 3 and 3-plus (1998 and later) use a transponder embedded in the key to communicate with the computer. New or non-original keys with incorrect resisters or transponders can trigger the security light, requiring that the driver follow a series of steps to tell the computer that the key is indeed valid. Contact a dealer to ascertain which system your van uses if you're not sure.
Insert the ignition key into the ignition, and turn it to the "on" position without starting the engine. Leave the key on for exactly 11 minutes. Make sure that the security light no longer flashes before proceeding to the next step.
Turn the ignition key off for exactly 30 seconds. Turn the ignition back on for another 11 minutes, and then back off for 30 seconds.
Turn the ignition key to the "on" position for another 11 minutes, then turn it off for 30 seconds for a final time.
Turn the ignition switch to the "on" position for 30 seconds, then attempt to start the engine. If the security light stays on or the van doesn't start or run, repeat steps 1 through 3. If that still doesn't work, contact a dealer to double-check that you have the correct information regarding your security system.
Turn the ignition key to the "on" position and connect a scan-tool to your OBD-II diagnostic port to erase any security codes stored in the computer. Any chain auto-parts store will have such a scan tool for public use; some charge for the service, others don't.
Attempt to start the vehicle without turning the key off from the "on" position. If the vehicle starts and then stalls, leave the key in the "on" position and wait for approximately 10 minutes while the computer relearns the key code. You'll know it has when the security light shuts off.
Turn the key to the "off" position and repeat steps 1 and 2 if the security light doesn't go off, if you had to turn the key to the "off" position at any point during the cycle or if the vehicle refuses to start or run.
Turn the key to the "off" position for 30 seconds, and repeat steps 1 and 2 again for a total of three relearn cycles. Turn the key to the "off" position, wait for 30 seconds and attempt to start the vehicle. The computer will recognise the key when the engine starts, and the security light should go out.
Insert a key into the ignition and turn it to the "on" position; the security light should illuminate and stay on.
Leave the key in the "on" position for about 10 minutes, or until the security light turns off. Turn the ignition key off for 10 seconds.
Repeat steps 1 and 2 two more times for a total of three cycles. After the last 30-second-off procedure, attempt to start the vehicle from the "off" position. The vehicle should start and the security light should be out.
If all else fails, you could try disconnecting the battery for 10 minutes. Depending on the model and the type of computer system, this might erase the computer's stored key memory and make it receptive to a non-original key.