Hi fellas, I've been a lurker on here for awhile, but decided to join the forum after buying a 2001 Pathfinder LE a few weeks ago. I know this is a typical question asked on here but I would appreciate your guy's help.
So I bought this Pathfinder a with "SES" light on. When I first got it, the vechicle would start up, but it would die unless you gave it gas for about 10 seconds, then it would idle low while in park. I went and got the code read at Auto Zone, and it showed that the IACV needed replaced. I bought a Duralast IACV from Auto Zone, and dropped the vehicle off to my local mechanic who has done work on my other vehicles over the years with hiccups. A fews days later my mechanic calls me and told me he was able to replace the IACV, but now when it starts up, in park it surges up to 1500RPMS, then after it has idled for around 1 minute, it goes back and forth between 1500RPMS to 2000RPMS until you put it in reverse or drive then it stays at 1500RPMS until you accelerate. He has tried to do the idle relearn on it, but it's not working and now the P0505 is showing back up saying the IACV is bad again. I was told that I just need to have my ECU repaired, but the ECU does not smell like it is burnt. I was also told that I need to have the ECU repaired, and after I get the ECU back, replace the IACV again, the TPS sensor, and then install the ECU and it should run normally again. Can any of you fellas help me out, or offer me some advice on what to do? I've alaways been a fan of the R50 Pathy's but if I'm heading for a money pit on this repair alone, could somebody give me a heads up please? Thank you.

The ECM will not damage the IACV, but the reverse can happen: the IACV can damage the ECM.
Mine needed to be repaired after the IACV went bad, even though visually I could see nothing wrong with the ECM circuit board.
No burning smell and no obvious component blown.
Don't use your nose to troubleshoot the ECM....lol.
No need to replace the TPS sensor either. That's just a red herring.
The relearn is a pain in the butt, but it needs to be done. That's probably the ultimate problem you're having now.
I couldn't do it successfully myself, so I ended up going to a tech at the dealership who did the relearn with his magic box in about 5 minutes.
He didn't charge me anything.

I agree in part with Buzzman. Be sure that the IACV relearn process is done right but first check to be sure the TPS is set right. The TPS is the only emissions control system component that is adjustable. There are two small screw that hold it in place. The screw holes are slots that allow for the necessary adjustment. If the TPS got clocked too far one way or the other, the system will not know where your actual idle position is. It's a simple check to be sure. You will need a multi-meter and feeler gauges. The TPS itself is a very basic component (like a simple potentiometer) that is not likely to go bad but very likely to be knocked out of proper adjustment.

The relearn is a pain the butt. You have to follow the FSM to the T. When it says drive the car for at least ten minutes to warm up the transmission fluid, you better do it. This was the part I was skipping and had the same idling issue. It wasn't until I followed the FSM to the letter, drove it for at least ten minutes (using a stopwatch!), did it finally take.