I’ve recently bought an 03 G35 Sedan and I’ve been having some issue with a P1615 code. The check engine light is NOT ILLUMINATED!! When I start the car it’s turnovers really slow as if the battery is dead however it isn’t I’ve replaced it with multiple and still have had the same outcome with EVERY single one. To add to that, every time I start the car the radiator fans immediately turn on and stay on until I turn the car off. I can start the car over and over but it just cranks slow before actually turning on. (Side Note: The NATS light doesn’t stay lit up when the key is inserted) I’ve done research on this and have found a mix of solutions, however no real answer! ANY HELP IS APPRECIATED THANK YOU ALL!!!

 

Maybe @seymore4 might have an answer.

 

really? has he done a thread on this ?

 

Sounds like you've got several issues going on, but P1615 is definitely NATS (the immobilizer). A NATS malfunction will give you a crank no start, but has nothing to do with how fast or slow it turns over. Sounds like an intermittent fault right now but it will get worse over time and eventually leave you 100% stranded. We can completely bypass NATS with an Uprev flash and make it a lot more fun to drive, fix the factory throttle restriction and really open it up. Even on a completely stock G35 the tune makes a huge difference.

Bigger issue is if your ECU is even tuneable or not, the early 03 sedans have an old version of the Nissan ECU that we cannot flash. Passenger side kickpanel, take a look and see which one you have.

 

In the last few days I got a new code and no longer p1615! I get P0605 now !! But the car starts right up and drives fine.