Welcome to 2CarPros. You replaced the ECM, did you have it programmed to match the 2000? It sounds like it isn't programmed properly for your truck and when the security system was programmed correctly it disabled it. To check it you need a scan tool that can read the security information and see what it is showing as a mis-match once it sends the start code.
What does the security light do when you turn the key? What does the truck do when you try to start it? Nothing, cranks but won't start? What if you spray a bit of starter fluid into the intake as you crank it? Do you hear the fuel pump turn on to prime the system?
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Tuesday, October 8th, 2019 AT 3:57 PM