Trouble Code: P0505 – Idle Control System Malfunction

The P0505  code Idle Control System Malfunction means that the engine computer cannot regulate the idle speed properly, or the actual idle speed varies from the target idle speed by a certain amount. The accompanied symptoms may include difficulty starting, hesitation, stalling at idle, fluctuating idle speed, as well as too high or too low idle speed.
There could be plenty of reasons for the P0505 code to set: vacuum leaks past the throttle, clogged idle air passages, bad or clogged idle air control valve, dirty throttle body, faulty electronic throttle, problems with the wiring or connectors, ECM problems, etc. Sometimes the carbon deposits in the idle air control valve can cause it to be stuck open or closed. Sometimes even low coolant level can cause the idle fluctuation and the P0505 code . Problems with the EGR and EVAP systems also can cause the P0505 code . If the idle air control valve or the throttle body is dirty, sometimes cleaning the either one can solve the problem. However, in many vehicles the idle re-learn procedure must be performed after the repair is finished. In some cars, the idle re-learn procedure is performed with a scan tool, in others the ECM re-learns the idle speed after the engine is run for some time.

How idle control works:

In modern cars, the engine computer (ECM) constantly adjusts the idle speed according to the conditions. It’s done by increasing or decreasing the air flow bypassing the engine throttle. You can notice the idle speed adjustment, for example, when you start the engine cold; the engine computer (ECM) increases the idle speed to help warm up the engine faster.
In some cars there is a separate idle air control (IAC) valve or solenoid (top photo) that controls the air flow at idle. In other cars the electronic throttle is used for the same purpose.
In a car with a separate idle air control valve, the throttle is closed at idle, so the only air supplied to the engine bypasses the closed throttle through the idle air passages and the idle air control valve, which is usually installed at the throttle body. The engine computer (ECM) adjusts the engine idle speed by slightly opening or closing the idle air control (IAC) valve.
If cars with electronic throttle idle control, the engine computer slightly opens or closes the electronic throttle to add or reduce the idle air flow.


– The diagnostic procedure for the code P0505 usually includes inspecting the idle air control system components, checking the IAC valve power supply voltage, inspecting electrical connectors and wiring, as well as testing the idle air control system with a scan tool. If beside the P0505 there are other trouble codes presented, they need to be looked at first.
– A Honda Technical Service Bulletin (TSB) dated August 2008 describes a problem with some early 00’s Odyssey and Pilot models where the stuck open idle air control (IAC) valve or broken power steering pressure switch wire can cause the idle to fluctuate, with the code P0505. As a solution Honda recommends replacing the IAC valve or (and) repair the power steering pressure switch wire located right above the steering rack boot on the passenger side of the vehicle.
– A Ford technical service bulletin for the 2006-2007 Explorer, 2004-2006 F150, as well as some other models describes a problem with hesitation, difficulty starting and low idle with the codes P0505 and/or P0506. Ford recommended reprogramming the ECM and replacing the electronic throttle body (ETB) if it fails the test as a solution.
– Another Ford TSB for the 2005-2007 Ford Five Hundred, Freestyle mentions spray cleaning the throttle body as a solution for RPM fluctuation with the codes P0505, P0506.
In some early 00′ Nissan and Infiniti cars with the code P0505 a bad IAC valve could even cause the engine computer to go bad as well.

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