Found this.
A "passive immobilization coil" was added around the barrel of the ignition switch to trigger a code from the remote handset when it is inserted in the ignition, enabling remobilization (see remobilization section below). Presumably the theory behind this "improvement" is that a thief cannot start the vehicle even if you forgot to lock it and he can turn the ignition switch on without a key. (NOTE: This wonderful feature also requires you to leave the key in the ignition when changing the battery to avoid immobilization.
Vehicle Battery Death While Vehicle Locked or Superlocked:
If you leave the headlights on or for some other reason the battery dies while your vehicle is locked or superlocked, strange things happen to the alarm system.
This seems to be one of the conditions that sometimes upsets the security system and requires you to use the Emergency Key Access method (see key/remote problems) to "reboot".
Another is when you jumper the battery to start the car, you can easily get locked out. When the power comes back on, the BeCM seems to try to return the vehicle to the locked state. At least one owner has reported that after gaining entry to his dead car with the manual key unlock, he put the key in the ignition and then went out and hooked up the battery jumper cables. Immediately, the vehicle locked him out. If all your keys are inside the vehicle at this point the situation is fairly bleak. So DON'T leave your keys in the vehicle if you get out. If you absolutely have to, at least leave the door or window open.
"Remote Battery Low" Message:
When you get the "Remote Battery Low" signal on the message display, take heed and renew the battery, especially if yours is a 1996 or later model with passive immobilization. On these models, if the battery dies entirely, the remote will not be able to transmit the code to the ECU to remobilize the vehicle and allow starting. (See procedure below under "weak reception from remote handset" before you give up though).
For battery replacement procedure see Key/Remote Problems and Solutions Page