Hello,
I am having an incredibly irritating issue with my V6 engine CEL not turning off and the code indicates that the IAC system has a problem with a P0505 code.
A couple years ago I had an experienced mechanic try to fix this. He replaced the IAC with 2 different new units and the code wouldn't go away. He checked and bypassed all of the sensors supposedly associated with the IAC, smoke tested the engine for leaks and there weren't any, even replaced the ECU and the code is still there. The light and code have been on for awhile, and the engine idles around 1100 to 1600rpm, but the truck runs and drives fine. This weekend I took a chance and got a new IAC from a Toyota dealer for almost $400, put it in, and the code is still there. I have also tried several training methods and nothing. I need this truck to pass Cal smog, they won't check the car because of the high idle and the CEL being on, and this problem won't go away.
Has anyone had something like this?
Thanks for any help.

 

Sounds like you may have the same issue: https://www.yotatech.com/forums/f2/e.../#post52484127

It seems like you have eliminated the IAC valve and the ECU as possible culprits, but what about the wiring from the IAC pins to the ECU? Something is causing that high idle, and its either wiring or sensors or bad connections. Checking the three wires for opens and shorts is the logical next step. Wiggle them a bit while you do these checks.

Other related posts suggest checking for vacuum leaks. There are several hoses on top of the engine that are potential vacuum leak sources basically any hose you see connecting to the throttle body or manifold: the PCV valve & hose, the power brake booster hose, the fuel vapor canister (black rectangular box to left of brake booster) has hoses and also two purge valve solenoids that can fail. I actually had one of these fail long ago but don't remember the symptoms. Most of these hoses can be disconnected and plugged temporarily to see if the high idle is fixed. In addition there are two vacuum actuators that look like silver flying saucers. The diaphragm in these can fail and leak vacuum. One is on the throttle body and one is hidden behind that (fuel pressure regulation I think). You can disconnect the hose going to these and suck on it to test if it holds vacuum. I think the fuel pressure one could actually leak fuel into the manifold if it failed so maybe test that last. I don't think the car will start without that working.

Some items pointed out here: https://www. you tube. com/watch?v=Sc1YMpujLXE