2003 monte carlo
 

I bought this car and for some reason there is a no crank no start. I know it's not the theft system. I checked all fuses and they are all good. Checked all relays with known goods and when I hooked up the scanner it wont connect to ecm. And i know the battery is good. Bought a new ecm and had it programmed and still no communication. I need some insight on what to do next.....thank you

This is going to sound crazy and extremely basic, but, check all of your positive, negative and ground connections. Your alternator, top and bottom under hood fuse box connections, battery terminals etc.. I upgraded the big 3-4 myself, then took my car to my mechanic for another issue, he didn't tighten the bottom connection on the fuse box, and my car started doing the same thing. He didn't charge me of course, but I spent a few weeks riding around getting stranded at random locations, all due to a loose connection.

I'm in the same boat as youwith an 03 ss , I took the starter to AutoZone since they test it for free, and the gear wasn't going up and down so I order a used one online and now waiting for it to come in. Could also be the solenoid connector wire. Have you checked your fuel pump or the fuel line's psi? , Double check the fuses because one time on my 01 Stang I got a mobile mechanic and he checked all the fuses and felt like they were all getting good voltage but I double checked with my grandfather and we found one faulty fuse it was a 10 fuse preventing it from starting up we put copper around it and it started up like before. So could be any little thing. I replaced the fuel pump and battery even though it has all the electronics working when I put it on acc but tryna see what happens when the starter comes in all I hear is some clicking in the fuses and the fuel pump whining. Hopefully it's not any wires that'll give me a headache.. but what I recommend you do is start looking at the starter, and all of those wires that connect to it as well check if they're getting spark look out for ANY loose wire under the hood and above it as well as the fuel pump and also the big 3, preferably with a reasonable mechanic that has experience