P0505 – Idle speed control (ISC) system -malfunction

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By Reinier (Contact Me)
Last Updated 2016-08-18
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0505 Idle speed control (ISC) system -malfunction
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Wiring, ISC actuator/IAC valve, throttle motor, throttle valve tight/sticking, ECM

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Table of Contents

  1. What Does Code P0505 Mean?
  2. What are the common causes of code P0505?
  3. What are the symptoms of code P0505?
  4. How do you troubleshoot code P0505?
  5. Codes Related to P0505
  6. Get Help with P0505

What Does Code P0505 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.

Idle Control Valve

What are the common causes of code P0505?

Although carbon build-up in the throttle body and/or idle control valve air passages is by far the most common cause of code P0505, other possible causes could include the following-

  • Engine vacuum leaks
  • Damaged, burnt, shorted, or corroded wiring and/or connectors
  • Open circuits
  • Defective idle air control valve
  • Defective step motor
  • Failed, or failing PCM. Note that PCM failure is a rare event, and the fault must be sought elsewhere before any controller is replaced.

NOTE: The idle control system on some GM products, most notably Dodge vehicles from the late 1090’s to early 2000’s, is particularly susceptible to damage caused by a defective charging system and/or low battery voltage. On these applications, a thorough inspection and testing of the entire charging system should always be the first step in diagnosing code P0505.

What are the symptoms of code P0505?

The symptoms of code P0505 are much the same for all applications, although the severity of one or more symptoms may vary between applications. Typical symptoms could include the following-

  • Stored trouble code and an illuminated warning light
  • Fluctuations in idling speed
  • Idling speed higher than specified
  • Idling speed lower than specified
  • Engine could stall unless idling speed is maintained by pressing the accelerator pedal

How do you troubleshoot code P0505?

NOTE #1: In many cases of code P0505, the problem can be resolved simply by cleaning out accumulated carbon deposits on, or in the throttle body and auxiliary air passages with an approved cleaner. Take note though that unapproved cleaning agents and chemicals can often damage internal surfaces and/or internal components.

NOTE #2: Defective alternators and/or batteries can have unpredictable effects on most applications, including generating random fault codes and damage to controllers. Since the idle control system is often one of the first systems to sustain damage caused by defective charging systems, it is important to check the condition, and functioning of the charging system before starting a targeted diagnostic procedure on the idle control system.

NOTE #3:  Be aware that some cheap generic code readers either cannot access the dedicated idle control diagnostic system that some manufacturers use to make diagnosing idle control system faults easier, or are not programmed with the dedicated software.  If a professional grade scanner is not available, the better option is to refer the vehicle for professional diagnosis and repair.

NOTE #4: Code P0505 is sometimes accompanied by several other engine codes and it is not always easy to determine if all, or some of the additional codes are the cause, or the result of code P0505. Thus, when diagnosing code P0505 and other codes, especially misfiring and vacuum system-related codes are present follow the rule that states that codes must be resolved in the order in which they were stored. Not doing this will almost certainly result in a misdiagnosis and a lot of wasted time.

Step 1

Record all codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

Step 2

If other codes are present along with P0505, diagnose and resolve them in the order in which they were stored. The most likely additional codes are usually vacuum system-related, so inspect the vacuum system for leaks caused by split, dislodged, hardened, or perished vacuum lines. Make repairs as required, and clear all codes before operating the vehicle. Retest the system to see if any codes return.

Step 3

If the code persists, remove the throttle body from the inlet tract and inspect it for the presence of excessive carbon build-up or deposits. In some cases it is possible to remove light carbon deposits from the throttle body and auxiliary air passages with approved cleaning agents; however, heavy deposits can usually not be removed without damaging the either the throttle body or the idle control valve. In these cases the better option is replacement of the throttle body and/or the idle control valve with OEM parts.

Step 4

When installing cleaned or replacement parts make sure all seals and/or gaskets are replaced as well to prevent vacuum leaks. Double check all vacuum line connections before starting the engine or operating the vehicle. If the code persists, consult the manual to determine the function, color-coding, routing, and location of all associated wiring. Perform a thorough visual inspection of all associated wiring; look for damaged, burnt, shorted, or corroded wiring and connectors.

NOTE: Engine wiring is exposed to higher temperatures than wiring elsewhere on the vehicle, which means that the insulation on engine wiring is far more likely to harden and flake off. This often causes intermittent short circuits, so resist the temptation to repair engine wiring, since merely moving the wiring can cause insulation to crack and split elsewhere. The better option is always to replace engine wiring harnesses with OEM replacements, as opposed to repairing them.

Step 5

If no visible damage is found on the wiring, perform resistance, ground, reference voltage, and continuity checks on all wiring in the associated circuit(s), but be sure to disconnect the idle control valve and step motor from the PCM to prevent damage to the controller. Compare all obtained readings with those stated in the manual, and replace wiring as required to ensure that all readings fall within the manufacturer’s specifications.

NOTE: Pay particular attention to the resistance of the signal wire between the PCM and the stepper motor, since even small variations from specified values can have serious effects on the step motor. Bear in mind that the step motor works in very small increments, which means that an incorrect resistance can seriously influence the working of the step motor, and therefore the amount of air that enters the engine during idling.

Step 6

If the resistance of the signal wire checks out, test the actual signal voltage that reaches the step motor at the connector. Use the scanner to command the idle control valve open, and check the actual voltage that reaches the connector; compare the obtained reading with the value(s) stated in the manual.

Note that depending on the application, dedicated idle control system diagnostic programs usually give a range of voltages that correspond to degrees of opening of the idle valve, or sometimes as degrees of rotation of the step motor. These readings should match the stated values EXACTLY- if they do not, replace the associated wiring harness with an OEM replacement.

If however, all obtained readings agree with stated values the step motor is defective, and it must be replaced. Bear in mind though that aftermarket replacement parts often do not conform to the quality standards demanded by car manufacturers, which could result in a continual recurrence of code P0505 (and others).  Also, keep in mind that in cases where the actual idle control valve and the step motor are separate components, the better option is always to replace both the motor and the valve.

Step 7

Clear all codes after all repairs are complete, and operate to vehicle to see which codes (if any) return.  At this point the engine should idle within specification, but if it does not, it is likely that there is an intermittent fault present. Be aware that intermittent faults can be extremely challenging and time consuming to find and repair, and in some cases it might be necessary to allow the fault to worsen before an accurate diagnosis and definitive repair can be made.

  • P0506Relates toIdle Control System RPM lower Than Expected”
  • P0507Relates to ‘Idle Control System RPM higher Than Expected”

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