Oh boy do I have an interesting problem. I'll start with a little background on what I've been working on. My buddy bought an 04 350z MT that needed the proper ecu put back into it. The reason being; it had an infiniti g35 AT ecu in it. Not sure how it was made to work, but the car ran and drove. It ran incredibly rich with this ecu in it, and unfortunately we live in a state that requires emissions. Otherwise we probably would have left it alone. We finally found the proper ecu that its supposed to have and installed it. I programmed the key and the car fired up and ran WAY better than with that g35 ecu. We took it for a test drive and everything was great until I tried to shift it into 5th or 6th. It won't go into 5th or 6th at all while its running. Thing is, it went into 5th and 6th no problem with the g35 ecu in it. I'm a little stumped as to why it won't go into 5th or 6th after an ecu swap. We also tried my old ecu from my 350z and it replicates the same issue. But when the g35 ecu is put back into the car, the gears are fine again. I'm hoping someone can help shed a little light on the subject or point me in the right direction

 

Are you saying this Z won't go into gear at all? Not clear on how the ECM can affect a mechanical linkage. Where in Colorado are you located?

 

Oh. No. 1st-4th are perfect. Smooth as butter. But 5th and 6th are the only gears it won't go into. My bad. I should have put that in the first post

We live in the northern Colorado area

 

There's nothing electronic in the clutch and there's no electronic gear lock-out. Have you ever pulled the shifter apart for any reason? Sounds like the reverse lock-out plate is misaligned.

 

We haven't done anything to the clutch or the shifter on this car. Reverse works completely fine too. Its only 5th and 6th thats the problem. And only after putting the correct ecu in it. Like I said, put that g35 ecu back in the car, all gears work again

The only thing we've done to this car so far is put the proper ecu in it

 

Might want to take a closer look at the gear linkage, as there's no electronic control of that, as others have said.