Working on a friend's 2004 G35 Coupe that's been having many, many... many issues; however our local auto shop thinks it's found the problem. The engine harness is very brittle, there are faults everywhere, and the ECM has been throwing codes left and right... but the components have been replaced.

We now think it's a shorted harness, that's 'fried' the ECM.
What's interesting, is that the O2 sensors installed when we got the car, have 6 pin connectors (2005-2006 year?), whereas it should have 4 pin connectors (2003-2004).

Additionally, the ECM currently installed do NOT match with Infiniti's VIN reference, as to what SHOULD be in the car.

We're thinking at one point in time, the wiring harness was bunk, so they installed an '05/'06 harness and ECM and O2 sensors, and the car ran fine.

Now, we are having many issues, so we are chasing after 2005/2006 harnesses and computers, so we can try the replacement.
The shop mentioned that we may have to bring it into Infiniti to have them reprogram it (?), but we are thinking we can do a direct swap with a harness/ECM we find off eBay/online.

Are we thinking reasonably or best case scenario?

 

Pull the ID number off the engine, might have been a complete transplant into an older chassis and they literally took the whole drivetrain and ecu.

 

ECM/ECU has: MEC35-250 C2 3X23
R1 on the lower left hand corner.
Unable to find an ID number on the engine.

That could be possible, but we're not sure.
We'd like to try a new ECM/harness at the same time; but maybe a harness first, then an ECM.
We're a little worried that with a new computer we may have to have Infiniti reprogram the computer to accept the old transponder keys, and since the ECM won't match the year of the VIN, it wouldn't work/they won't honor the work.