Ok I'm been fighting this one for awhile now. Code P0505 IAC circuit problem. I replaced the iac no help. I checked voltage with the IAC unplugged and the truck running, I got 1.8 volts on 2 wires that cycled off and on. I checked for resistance between the IAC plug and the appropriate pins on the C1 connector and have minimal resistance. I'm thinking that my computer is bad but I don't want to buy one for no reason. Side symptoms are the fuel gauge doesn't work, reverse lights don't work and the transmission output shaft sensor is throwing a code but the speedo is still working. Any ideas? Am I on the right track? If it is a computer who should I buy from other than the dealer and who should I avoid?

 

Don't rush into the computer. Take one step at a time.

IAC is a stepping motor. Did you disconnect the battery when working on this? Do that and let set, then reconnect and try. Make sure OAC port is clean - really clean! The IAC voltage was cycling off and on as that is how a stepping motor is controlled. Each voltage step moves it one step.


Reverese lights are a switch on tailshaft of tranmission. That is the trans output shaft sensor.

Seems computer is working if it is identifying problems.

 

Maybe pull the IAC and try cleaning the carbon buildup on pintle and the port it steps into, with neg pulled off battery, before doing anything with sensors. Do not push or twist on pintle while cleaning After reinstalling go out on the highway and get up to 50 mph and let off gas and coast to a stop a couple times for it to relearn position faster.

 

My thinking is that the computer isn't sending enough voltage to drive the IAC since it's only sending 1.8 volts instead of 12 volt. I did the reset with disconnecting the battery and turning the key on and off 10 times before starting it also did the accelerate and decel method. I put a new IAC in and the idle was too high even after doing the reset. So I cleaned the old one and reinstalled it to get the idle down. I've been chasing this for awhile.

 

Check connections at PCM as well. Sounds like you just have a poor connection somewhere.