To reinforce what forum member lout has indicated in post#2, attached are some excerpts from the Holden VT/VZ Service Manual that support your problem being immobiliser-related. Note the highlighted sections:
VY THEFT DETERRENT SYSTEM OPERATION
V6 AND V6 SUPERCHARGED ENGINE
When the ignition is turned on, the BCM polls the PCM and sends an encrypted BCM / Key security code. The security code is received via the slip ring, or by the remote receiver in the event of no slip ring communication. The PCM compares the received security code with its stored security code and if the codes match, the PCM enables injector fuelling and continued engine cranking. The PCM returns an OK TO START message telling the BCM to jump from SHORT LOOP mode to the LONG LOOP mode.
Armed
The theft deterrent system can be armed actively, by pressing the lock button on the remote coded key or passively, when the BCM automatically arms the system 30 seconds after the ignition is turned off.
When the theft deterrent system is armed, the following components are disabled preventing the engine from being started:
• The start relay located in the engine compartment relay housing.
• The PCM.
• The PIM (on vehicles with GEN III V8 engines).
Disarmed
The theft deterrent system can be disarmed by pressing the unlock button on the remote coded key (which will unlock the doors, turn the interior dome lamp on and disarm the system for 30 seconds) or by inserting the remote coded key into the ignition switch and turning the ignition on.
With the system disarmed and the PCM / PIM enabled, the engine can be started.
The BCM then reads a security code (serial data output) from the remote coded key contact pin via the remote coded key reader assembly.
NOTE 1: Both the V6 and V8 systems allow up to 1 second of cranking time if the code is invalid. This usually results in the engine starting but then dying after two to three seconds. Normally this allows for fuel priming and reduces start delays while the security message is being validated. Engine crank is then disabled until the next ignition cycle of key off - key on.
