Have changed the ecm, changed coil packs, put code reader on and it said car not responding.
This may be the dreaded antitheft security system. Look for a picture on the dash when this happens that says SECURITY or a picture of a car with a lock through it. This 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, or not seen at all. When the communication breaks, the car misinterprets this as an attempt at theft and goes into antitheft mode. (like it thinks it's being hot wired) The car shuts down because it is not getting that signal from YOUR specific key. This is why the security light turns on and shuts down your vehicle. It will either turn off the starter, or the fuel injectors. The vehicle may run for 2-3 seconds before shutting down. This link explains it pretty well.
http://www.diy-gm-security.com/
Does your key have a chip on it ? if it does , the problem is going to be that ,key chip only works with the original computer , until you have it reprogrammed by the dealer . this is an anti theft function to keep someone from stealing your car . there fore your car will not start until the chip matches the ecm.
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