What's the difference between P0505, P0506, and P0507?

All three are idle control codes, but they describe different failures. P0505 is the general 'idle air control system isn't responding correctly' code — the PCM can't get the IAC to produce the expected idle behavior. P0506 means idle is consistently too low (RPM lower than commanded), which usually points at a stuck-closed IAC or a restricted air passage. P0507 means idle is consistently too high (RPM higher than commanded), which usually points at a vacuum leak or a stuck-open IAC. P0505 is the umbrella; P0506 and P0507 are the specific failure directions. You'll often see P0505 + P0506 or P0505 + P0507 together, with the second code narrowing down the cause.

Can I keep driving with P0505?

Yes, it's not a safety code. The engine still runs and drives fine — the IAC's job is only at idle. What you'll experience is annoying rather than dangerous: rough idle at stoplights, occasional stalls when coming to a stop, and potentially harder warm restarts. You can drive the vehicle for weeks like this if needed. The bigger concern is that an unstable idle can mask other issues (vacuum leaks, sensor problems) that might develop into bigger problems. Get the diagnosis done within a month or so.

Should I try cleaning the IAC valve first?

Yes — cleaning is the cheap test that resolves about half of all P0505 codes. The IAC valve and the air passages around it accumulate carbon and oil mist from the PCV system, and the buildup eventually keeps the IAC from moving freely. Remove the IAC (usually 2-3 small bolts on the throttle body), inspect for visible buildup, and clean both the valve and the bore with throttle body cleaner. Reinstall, perform the idle relearn procedure for your vehicle, and see if the code stays gone. If the symptom returns within a few weeks, the IAC is mechanically worn and needs replacement.

Why doesn't P0505 apply to my newer car?

P0505 is specific to vehicles with a dedicated idle air control valve — a separate component that bypasses air around a closed throttle plate to maintain idle. On vehicles with electronic throttle control (most cars built after about 2005-2008), there's no separate IAC valve because the throttle plate itself opens slightly at idle to do that job. The PCM commands the electronic throttle to whatever position maintains target idle. If a scanner shows P0505 on a modern electronic throttle vehicle, it's most likely a code translation error or a generic scanner showing a code that doesn't apply to the platform. On electronic throttle vehicles, the equivalent failures show up as P0121, P0122, P0123, P2135, or related throttle codes — not P0505.

OBD-II trouble code

P0505: Idle Air Control System Malfunction

The idle air control (IAC) valve isn't responding to PCM commands the way it should — idle is rough, hunting, stalling, or running too high. Applies to older pre-electronic-throttle vehicles where a dedicated IAC valve controls idle airflow around a mechanically-cabled throttle.

Quick facts

System: Powertrain
Category: Throttle / Idle
Severity: Medium severity
Drivable: Usually safe to drive short-term
Repair cost range: $50 – $400
DIY difficulty: Intermediate DIY

What does P0505 mean?

P0505 is a code from the pre-electronic-throttle era of automotive engineering. On vehicles with a traditional cable-operated throttle (most cars built before roughly 2005, plus some commercial and economy vehicles built later), the throttle plate is held closed by the return spring when the driver isn't pressing the pedal. To keep the engine idling, the PCM has to allow a small amount of air to bypass the closed throttle plate. The idle air control (IAC) valve is the part that does this — a small stepper motor or solenoid that opens a calibrated bypass passage around the throttle plate to feed the engine just enough air to maintain target idle speed.

P0505 sets when the PCM commands an IAC position and the resulting idle speed doesn't match what's expected. Either the IAC isn't moving (stuck), isn't sealing (leaking past), or isn't responding at the speed the PCM expects. On modern electronic throttle vehicles, the throttle plate itself opens slightly at idle to do this job — there's no separate IAC valve — so P0505 doesn't apply. If you see P0505 on a vehicle built after about 2008, it's most likely a misapplication or a translation error from a generic scanner.

What the driver experiences is rough or unstable idle, idle that hunts up and down, stalling when coming to a stop, idle that's stuck too high or too low, or hard restarts after a warm shutdown. The engine still runs and drives fine off-idle because the IAC's job is only at idle — once the driver is on the throttle, the main throttle plate is doing the work.

The leading cause of P0505 is the IAC valve itself failing or being so gummed up with carbon and oil that it can't move freely. The second cause is a large vacuum leak that makes the PCM unable to control idle even with the IAC fully closed. The third is wiring or connector issues at the IAC.

Common causes

Carbon and oil buildup inside the IAC valve preventing free movement
Failed IAC stepper motor or solenoid — won't respond to commands
Carbon buildup in the idle air passage around the throttle plate
Vacuum leak large enough to overwhelm idle control (intake gasket, brittle vacuum hose, leaking PCV)
Damaged or chafed IAC wiring
Corroded or pushed-back pins in the IAC connector
Failed throttle position sensor providing wrong baseline to the PCM
Failed coolant temperature sensor (the PCM uses coolant temp to set idle target — bad CTS can confuse idle control)
Failed PCM driver circuit (rare)

Symptoms

Check Engine Light on
Rough or unstable idle
Idle hunting up and down (RPM cycling between 600 and 1,200)
Stalling when coming to a stop
Idle too high or too low and not responsive to load (AC on, headlights on, etc.)
Hard restart after warm shutdown
Engine may run fine off-idle but be miserable at stops

Diagnostic steps

1.Confirm the vehicle actually has a separate IAC valve. On most vehicles built after about 2005-2008, idle is controlled by the electronic throttle plate, not a separate IAC. If your vehicle is post-electronic-throttle, P0505 is likely a misread or a translation issue.
2.Visually locate the IAC valve — typically a small unit bolted to the side or top of the throttle body, with a 2-pin or 3-pin connector.
3.Remove the IAC valve and inspect. Heavy carbon buildup in the valve body or air passages is your culprit. Clean with throttle body cleaner and reinstall.
4.Test the IAC electrically. Most IAC valves are stepper motors that can be tested with a multimeter for coil resistance to spec.
5.Smoke test the intake for vacuum leaks. A large leak can cause P0505 even with a healthy IAC because the PCM loses control authority.
6.Inspect the wiring and connector for damage, corrosion, or push-back.
7.If the IAC tests bad electrically or doesn't respond to commands after cleaning, replace it.
8.After replacement, perform the platform-specific idle relearn procedure.

Repair cost

$50 – $400

Low end is IAC cleaning — $10-20 for a can of throttle body cleaner plus an hour of work. Often resolves the symptom. Mid-range $100-200 covers IAC replacement on most platforms — the part itself is $50-150 on common platforms and labor is usually 30-60 minutes. Upper end is $250-400 for IAC replacement on harder-access platforms or where vacuum leak repair is needed alongside the IAC work. If cleaning resolves the issue but the code comes back within months, the IAC is wearing out and replacement is the lasting fix.

Estimate your repair

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Open the Repair Cost Estimator with idle air control valve replacement preselected. Adjust labor rate and vehicle category to fit your situation.

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DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.