Glad to be of help. First thing is to have your battery checked. You should have at least 12.6 volts at the battery with engine off. If battery is good, turn key on without starting, leave on for 10 minutes before starting. Try this first.
The most common intermittent no start problem I am aware of on this system is in the ignition lock and housing.
This system works by the sensing of the ignition lock being rotated. The ignition lock in this vehicle is open. It has been very common to have excessive grease (graphite) in many cases to spray in the housing blocking the magnetic field. Thus, the computer cannot sense the rotation of the ignition lock.
There is no special ignition key and the security is built into the lock and the housing with a magnetic field. The vehicle's computer senses the rotation of the lock and thus fuel is delivered.
Here is the passlock key relearn procedure:
Theft deterent systems, relearn procedures
Passlock III Relearn Procedure
30-Minute Relearn Procedure
Use this procedure after replacing any of the following components:
The PASS-Key III plus keys
The theft deterrent module (TDM)
The powertrain control module (PCM)
This procedure is not available on vehicles equipped with EXPORT configured TDMs.
When performing the 30-minute relearn procedure, all previously learned keys will be erased from the TDM memory. Additional keys may be learned after performing the relearn procedure.
When replacing a TDM with a GM SPO Replacement Part, ensure the procedure to setup a new TDM is performed prior to the 30-Minute Relearn Procedure.
•4. Repeat steps 1-3 two more times for a total of 3 cycles or 30 minutes.