What does it mean?
OBD II fault code P0505 is defined as “Idle Control System Malfunction”, or sometimes as “Idle speed control (ISC) system –malfunction”, and is set when the PCM (Powertrain Control Module) is unable to control, or maintain the engine idling speed at a specified RPM. Note that regardless of slight differences in the wording of various definitions of code P0505, this code always refers to the PCM’s inability to maintain or control the engine idling speed.
On most applications, the PCM will attempt to make adjustments to various systems such as fuel trim, ignition timing, injection pulse width and others in attempts to maintain the specified idling speed and quality. However, PCM’s generally have a rather limited ability to correct or compensate for most faults, and when this limit is reached the PCM will set code P0505 and illuminate a warning light.
Idling speed on gasoline engines is regulated and controlled by the idling control valve (generally located on the throttle body) that allows air to enter the engine by by-passing the throttle plate, which is for all practical purposes kept almost closed by the PCM during idling.
A sufficient supply of air to the engine during idling is accomplished by an idling control valve that is operated by a step motor that adjusts the amount of air entering the engine in small increments or “steps”, hence the term “step motor”. Increments are determined by the PCM, and are based on information obtained from several sensors scattered around the engine.
Typical information used by the PCM to control the idling speed include- engine temperature ambient air temperature fuel pressure fuel temperature/composition ignition timing signals transmitted by the Crankshaft Position sensor valve timing data as transmitted by Cam Position Sensors data provided by the Throttle Position Sensor data from the transmission data from oxygen sensors, and data about which electrical consumers such as A/C, wipers, and headlights are in operation that place an additional load on the engine.
As long as the throttle plate remains closed, the idling speed and quality is controlled by the PCM via the idle control valve, and as can be seen from the above it should be clear that idle control on a gasoline engine is a complex affair.
For instance, if the engine is started in sub-zero temperatures, the PCM will command the idling control valve to allow less air to enter the engine to enrich the air/fuel mixture to create a condition which is roughly analogous to how chokes worked on carburetted engines to control idling in cold weather.
As the engine heats up and combustion becomes easier and more complete, the PCM will command the idling control valve to increase the amount of air entering the engine to suit the changing conditions. However, when the load on the engine changes, say, when some electrical consumers or other systems are either turned on or off, the PCM adapts the signal voltage to the idle control valve to ensure that the idling speed remains stable, and at the RPM specified by the manufacturer.
One example of this would be when an automatic vehicle in placed in gear. To prevent the added drag from the torque converter and transmission from reducing the idling speed, the PCM will command the idle control valve to allow more air to enter the engine, while at the same time, adjusting the injector pulse width to inject more fuel to suit the increased amount of air.
Note that the idle air control valve is closed by the PCM when the throttle plate is opened to prevent excess air from entering the engine. The image below shows the typical location of the Idle Air Control Valve. Note that the appearance and location of Idle Air Control Valves differ between applications; consult the relevant manual to locate the Idle Air Control Valve on any particular application.