I blew my head gasket so I decided to just get a used engine.. while my father was talking out the engine he disconnected the harness wire without unplugging the battery and now while I replace the engine I purchase my ride will not start auto theft light is on I tried reprogramming it but the computer don't seem to catch it what can I do I've heard that you're not supposed to disconnect the harness wire until you disconnect from the battery how can I fix that problem now...

 

Scan it for fault codes.
Are you using your original ECU/PCM?

 

I have a mazda 3 2005. At this time of year whenever the weather gets cold (below 10 degrees C) it refuses to start and the auto theft light stays on. What I do to get the car to start is put the key in the ignition and turn it to position 2. Leave it for 5 mins. It must warm the ignition system. The car always starts after that. I've had the problem for a few years. It is too expensive to fix, but what I do ALWAYS works. I haven't replaced the engine or done any major work, so this fix might not have any effect. It's worth a try, no costs.

 

Yes I have my original ECU/PCM my AT is on.. can a PCM be replaced by a use one well that Even work

 

Yes you can use a second hand PCM, however it will need to be reprogrammed to your car & your keys.

Best next step is to scan for codes.