P0506 – Idle speed control (ISC) system -rpm lower than expected

By Benjamin Jerew (Contact Me)
Last Updated 2016-09-24
ASE Master Tech

Trouble CodeFault LocationProbable Cause
P0506 Idle speed control (ISC) system -rpm lower than expected Wiring, ISC actuator/IAC valve, throttle motor, throttle valve tight/sticking, ECM

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What Does Code P0506 Mean?

While driving, your foot on the accelerator pedal effects a change in throttle opening, which, in turn, effects a change in engine power and speed. When you release the accelerator pedal, the throttle closes, returning the engine to idle speed. To maintain engine idle speed, the IAC (idle air control) valve allows a small amount of air to bypass the throttle valve. Some manufacturers put the IAC in the throttle body, while others may place it elsewhere. Many modern vehicles, drive-by-wire vehicles with an electronically-controlled throttle body, do not employ a separate IAC, but control idle speed by varying throttle opening directly.

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The ECU (engine control unit) modulates IAC valve function to control idle speed, using the CKP (crankshaft position) sensor for feedback. At any particular IAC value, whether opening percentage or duty cycle, the ECU expects the engine to be running at a specified rpm. The ECU also modulates IAC to change idle speed, depending on engine load, such as high-idle when the air conditioning compressor is activated or the power steering pump is active.

If the ECU detects a problem in the system, such as engine idle speed being higher or lower than expected for the IAC opening, it will set a DTC (diagnostic trouble code) and illuminate the MIL or CEL (malfunction indicator lamp or check-engine light). If engine rpm is lower than expected, the ECU will set DTC P0506 – Idle Control System RPM Lower than Expected in memory.

What are the common causes of code P0506 ?

Depending on year, make, and model, DTC P0506 may have number of causes. Here are some of the most common.

  • ECU Update – Newer vehicles, particularly those with drive-by-wire electronically-controlled throttle bodies, may not be equipped with an individual IAC, but use the main throttle valve to maintain idle speed. With or without an IAC valve, always check with the manufacturer for possible recalls, TSBs, or updates, that may address idle problems.
  • Stuck IAC – The IAC valve is a sensitive valve, and the tiny motor or solenoid that operates it isn’t particularly strong. Over time, carbon deposits can obstruct the movement of the valve. Regular cleaning can prevent this, but very dirty valves may need to be replaced. Don’t forget to replace the gasket, if applicable, when replacing the valve.
  • Stuck Throttle Valve – Similarly, carbon deposits can easily obstruct the movement of the throttle valve. Regular cleaning is recommended, and the throttle body is far more resilient than the IAC. Replacement isn’t usually required, but check with a professional for a firm diagnosis and possible TSB updates.

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The following idle control system problems would most-likely be accompanied by fuel system lean codes, which should help in narrowing down your diagnosis.

  • Vacuum Leak – Over time, rubber hoses tend to become brittle, especially in the heat of the engine compartment. There may be dozens of vacuum lines controlling a number of different systems, from the EVAP system to the air conditioning system. Often, vacuum lines tend to break right where they connect to the nipple, and you can simply trim off the broken piece and reconnect it. If the hose is brittle, however, you may need to do a full replacement.
  • PSP Idle-Up Valve – Many power steering systems include an idle-up VSV. When power steering pressure is detected by the valve, it opens a by-pass, allowing more air past the throttle body and increasing idle speed a couple hundred rpm. A broken valve may simply let air through all the time.
  • Defective PCV Valve – If the PCV (positive crankcase ventilation) valve is stuck open, it will apply manifold vacuum to the crankcase, which may start sucking in air almost anywhere, such as from the dipstick tube or valve cover gasket.

What are the symptoms of code P0506 ?

Typically, engines idle from 400 rpm to 800 rpm, depending on the vehicle, perhaps higher if it is high-idling for air conditioning or engine warmup. If you are having a problem with the IAC system, and the MIL is on, you may also notice the engine idling unusually high, or erratically idling high and low. You may also experience hard engine starting.

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How do you troubleshoot code P0506 ?

When diagnosing DTC P0506, you’ll want to focus on the air intake system, particularly the throttle body, IAC, and intake manifold. Pay attention to concurrent DTCs, which could also point you to a solution. Recalling that the ECU commands the IAC to open and close, varying the amount of air bypassing the throttle valve, to maintain engine idle speed should tell you that you have an air problem, but you shouldn’t rule out electrical or mechanical problems. On newer vehicles, call the dealership and ask if any TSBs (technical service bulletins) apply to your vehicle and issue.

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  • Throttle Inspection – Make sure the throttle valve is clean and operating properly. On both mechanical and electrical throttle bodies, make sure the valve opens and closes with no resistance. Carbon buildup in the valve or a sticking throttle cable can easily hold the valve open, allowing more air passage than the ECU is expecting. A dirty throttle valve can easily be cleaned with carb-cleaner. Replacement is usually not necessary.
  • IAC Inspection – Similarly, make sure the IAC valve is working properly.
    • Make sure the IAC properly connected and there are no broken or bent pins or corrosion.
    • You can test IAC operation by turning on the air conditioning or turning the steering wheel all the way to one of the locks. Either case should cause the ECU to request an idle-up. If not, suspect a stuck valve, open solenoid, or wiring problem.
    • You may get a hint of IAC operation if you noted carbon deposits in the previous inspection. Remove the IAC valve if you have to, and attempt to move the valve using a toothpick. If the valve is stuck or moves with difficulty, try to clean it with carb-cleaner. You can also bench-test the valve at this time to verify proper function. Extremely dirty IAC valves may require replacement.
  • Intake Inspection – Check for vacuum leaks.
    • Check for cracked or broken vacuum lines after the throttle body, such as those going to EVAP or PSP VSV (evaporative emissions or power steering pump vacuum switching valve).
    • When testing IAC function with the steering wheel, be sure to check that the PSP VSV is functional. You can check this by plugging or pinching the inlet VSV inlet hose or filter and seeing if the idle speed changes. With the power steering system inactive, no idle-up should be expected.
    • Check intake gaskets for leaks, as well.
  • Other DTCs – Some DTCs may point you to the solution. Vacuum leaks could cause high-idle condition, as well as fuel system lean condition, which would set DTC P0171 or P0174 in memory.

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Codes Related to P0506

While diagnosing DTC P0506, you may note concurrent codes, such as:

  • P0171 – Fuel System Too Lean (Bank 1)
  • P0174 – Fuel System Too Lean (Bank 2)
  • P0505 – Idle Control System Malfunction
  • P0508 – Idle Control System Circuit Low
  • P0509 – Idle Control System Circuit High

Similar to DTC P0506, but at the opposite end of the scale, is:

  • P0507 – Idle Control System RPM Higher than Expected

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