First, make sure you look at the Nissan Maxima as it's pretty close mechanically.

The 2001 I30 (and Maxima) has a critical problem that affects vehicles as they get more miles and approaching/over 200,000 miles. The symptoms are related to stalling as well as a P0505 Check Engine light and erratic idle.

My understanding of the cause is that coolant runs through the Throttle Body (by design to help "warm" during extreme cold) and that wears down the seal in the Idle Controller. Eventually, it leaks from the seal and damages the stepper motor. This in turn sends an invalid signal/power surge to the ECU/ECM which (as is not protected by a fuse) in turn, damages the ECU.

It was quickly and accurately diagnosed by the Nissan Dealer (as its a VERY common problem) and their solution was to cost $2,200 for a new/reprogrammed ECU and Idle Controller (which alone was quoted at $650 total).

For a high mileage i30/Maxima, this cost is likely a death sentence for the vehicle and I'm sure there are many junked or selling at auction with this condition.

The fix is somewhat involved, but relatively straightforward. There are numerous companies out there that will fix your ECU for under $100 within 24 hours of receiving it (new MOSFET chip) and removing the ECU is straightforward, although involves accessing 4 difficult 8mm bolts and an awkward position to get to it. No ECU should be installed PRIOR to having the IACV fixed or there is likelihood of repeat failure.

The second part of the fix is more time consuming. The Airbox needs to be removed as well as the throttlebody which includes: 4 primary bolts, 2 throttle body cables, plus 2 hoses which have likely fused themselves to the IACV. Once removed, the new part (WITH new gasket) can be installed and the throttle body should be cleaned on both sides to eliminate potential sticking throttle or other rough idle conditions.

Note that I have not completed the repair yet (still trying to remove one more hose) and while straightforward has taken me a lot of time to get the 15 year old parts off.

Repairing the ECU is recommended as otherwise, the new ECU will need to be programmed at the dealership with all keys.

A competent mechanic might be recommended instead of the dealership, but enure again that the vehicle is not attempted to be started with a good ECU and bad IACV or the ECU will likely be damaged again.

It should be noted that other common problems with the i30/Maxima related to idling include cleaning/replacing the Mass Air Flow sensor (in that airbox) as well as a dirty throttle body. It may be worthwhile to go ahead and replace the MAF at the same time since the airbox will be removed.

Only use Genuine Nissan parts for this/these repairs as these are complex electronics. New Nissan IACV costs $200 online and a new Nissan MAF costs under $90. ECU repair is under $100.