Theft light stays on and the vehicle shuts on and off because of this

Anti theft warning light.

Anti-theft system problem , possibility ! Do you have a security light blinking ? Security Indicator
The security indicator is located in the clock on the top of the instrument panel. The light emitting diode (LED) displays the immobilizer system status. The immobilizer contains the LED driver hardware for direct connection of the LED.
Vehicle Theft Deterrent (VTD) Operation
The immobilizer system design prevents vehicle theft by interrupting the engine operation. The immobilizer system prevents the vehicle from starting. The engine operating functions interrupt if the immobilizer system does not recognize the transponder identification code of the ignition key. The immobilizer system and the content theft deterrent system do not interact. The system includes the following components:
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The ignition key

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The column-mounted theft deterrent module with integrated antenna

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The transponder

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The engine control module (ECM

You then have a ant-theft system problem an need to have the vehicle checked for DTC'S - diagnostic trouble codes . The ASD - relay supplies power to the ignition coils an too the fuel injectors .The ASD- auto shut down relay is energized by the engine computer - PCM . With the red anti-theft light on the PCM will not turn on the ASD relay cause a no start condition . You can check fuse an relays all you want ,not going to do any good . You need to see what the ant-theft systems problem is .

Is the security light on or flashing when trying to start the vehicle .
Anti-Theft The passive anti-theft system (PATS) consists of the following components:

• Anti-theft indicator
• Encoded ignition key(s) (the key contains a transponder)
• PATS transceiver
• Powertrain control module (PCM)

PATS will disable the vehicle from starting if there is:

• a damaged PATS key.
• an unprogrammed PATS key.
• a non-encoded key (a conventional key or one that does not have any electronics).
• damaged wiring.
• a damaged transceiver.
• a damaged PCM.

If it is a anti-theft system you will have to take it to the dealer .This system will keep the starter from spinning the engine .

you could have a few problems,check the "auto shut down relay " that is in the pdc ( fuse box) under the hood several of them are the same so substitute the A/C relay and see if that works, Or you might have an issue with the SKIM module that reads the key if it does not read a programed key it will run for a few seconds and shut the fuel off to prevent theft-is there a little red light on in the dash? thats a security light for the theft system

The fog lamps are computer controlled .the body control module (BCM) will energize the fog lamp relay control circuit. The BCM receives the park lamp ON signal from the interior park lamps supply voltage signal circuit.
DTC B2527 Front Fog Lamps Circuit Low
The body control module (BCM) monitors the front fog lamp switch signal circuit in order to determine when the fog lamp relay is to be energized. When the BCM senses that the front fog lamp switch signal circuit is grounded momentarily by pressing the fog lamp switch, the BCM energizes the fog lamp relay by grounding the fog lamp relay control circuit.

Your best bet , take it to the dealer . Could need BCM replaced , would need to do testing using the Tech 2 scan tool . A new BCM has to be programmed .

Body Control Module (BCM) Replacement
If the body control module (BCM) is replaced, the following procedures must be performed in the following order:
BCM Reprogramming--Refer to Body Control Module (BCM) Programming/RPO Configuration in Computer/Integrating Systems.
Important: This procedure has to be performed only for vehicles equipped with the Passlock™ System.
Theft Learn--Refer to Programming Theft Deterrent System Components in Theft Deterrent. A new BCM will learn the Passlock™ information automatically, but the powertrain control module (PCM) must still learn the theft information from the BCM.

The security, antitheft, or passlock light is a visual indication that something in the car's antitheft system is failing. In most GM cars and trucks there is a hidden system that most people are unaware even exists. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM (Theft Deterrent Module). These 3 parts are in CONSTANT communication. If the key is in the ignition, then these parts are sending signals to each other. The security light illuminates when that signal gets broken, even just for a second. When the signal breaks, the car misinterperates this as an attempt at theft and goes into antitheft mode. The car thinks its being stolen or hotwired because it is not getting that signal from YOUR specific key. This is why the security light turns on and shuts down your vehicle.
This link may help you find ways to bypass this or fix it yourself.
http://vatspasslockpasskeysecurityhelp.yolasite.com/

When the motor was replaced , was the original PCM changed as well ? Did you have the anti-theft system programmed . Theft - system light on -DTC'S - diagnostic trouble codes are stored . Need a factory or professional scan tool read these codes . The other computers on the vehicle will communicate . BCM
Important: Due to component variability, the vehicle theft deterrent (VTD) system must have the learn procedure performed regardless, if the vehicle starts on the first ignition cycle after a VTD repair.
All codes in the theft deterrent module must be cleared for a relearn.
The vehicle theft deterrent system is designed in order to prevent vehicle theft by disabling the fuel injectors unless the lock cylinder is correctly engaged by a mechanical key. The theft deterrent system uses the following 4 components for theft prevention:
• The lock cylinder
• The ignition switch
• The body control module (BCM)
• The powertrain control module (PCM)
When starting the engine, the PCM searches for a password from the BCM through the Class 2 serial data circuit. If the password is not recognized or not present, the PCM will disable the engine. Two modes of tamper detection are provided:
• No password received
The engine will start and stall quickly. SECURITY telltale will flash on the instrument panel cluster (IPC) and then stay ON steady.
• Incorrect or disable password received (More than 3 invalid passwords are received)
The engine is disabled for at least 10 minutes and the SECURITY telltale will illuminate solid on the IPC during the 10 minutes.
After the vehicle has passed theft detection, the PCM will continue normal operation.

DTC B2947 Security System Sensor Power CKT Low
DTC B2948 Security System Sensor Power CKT High
DTC B2957 Security System Sensor Data Circuit Low
DTC B2958 Security System Sensor Data Circuit High
DTC B2960 Invalid Coded Key Present
DTC B3031 Key Decoder Device in Assem Learn Mode
DTC B3033 Security System Indicates Tamper

Does pin # 16 at the DLC have B+ voltage ?

Alot of the time, the Anti theft that came stock on your vehicle can shut the car down. When the system starts to get worn down, the car tends to mis communicate and shut itself off. This is to prevent theft. #1. Is there a light somewhere on the dash when this happens? Should say security or anti theft. If you see this light, then this is 100% your problem. This link explains the situation and offers some proven DO IT YOURSELF solutions. Hope this helps. http://vatspasslockpasskeysecurityhelp.yolasite.com/

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