Ihad another mechanic come take a look at it and plugged his computer into it and it threw him the code U1000 on my 2004 g35 sedan and said that I need a new ECM. All ground wires were good, battery is good, starter is good. I just need an opinion if yall really think it could be the ECM that's wrong because I don't want to buy it and find out that it wasn't the problem so I need an opinion. Also I’ve seen it could be NATS or maybe even key reprogramming. The mechanics that looked at it are mobile mechanics. Do yall think I should take it to a professional mechanic shop or the dealership?

Also the security light stays on while on ignition. I’m not sure if that’s normal or not.

Previously...

04 g35 sedan. It's a no crank no start I bought it like that 4 weeks ago and old owner went to get his starter changed and they also put a different battery to see if it would start and it did not. He also changed alternator as well and nothing as well. The mechanic told him to buy a new fuse box ans that they were going to put a new one in ans he bought it but mechanic never came. Today I had a mechanic come look at it he tested previous fuses and put in the new box and he tested the starter and fuses they were all good. і changed the battery he also tested it and its good. also the fuses from the battery cable were good. I tried starting the car in Neutral but didn't do it.