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P0505 points to the idle air control (IAC) system - the mechanism that holds your engine at a steady idle. On older throttle-body engines this is a physical IAC valve; on drive-by-wire systems it's controlled through the electronic throttle. Carbon buildup is the leading cause and cleaning often cures it. See top-rated scanners on Amazon ↗
🗺️ Where Is the Problem?
Blueprint view - P0505 idle air control malfunction; IAC valve or throttle body at fault
These are statistical causes across ALL vehicles - your exact car may rank differently
For example, on a Honda 4-cyl the downstream O2 sensor causes P0505 64% of the time, but on a GM 5.3L V8 the catalytic converter is the cause 71% of the time. Get a probability ranking built specifically for your year, make, model, and mileage.
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🎯 Top Causes & Probability
55%
#1 - Most Likely
Dirty or Failed IAC Valve
The idle air control valve regulates airflow around the throttle plate to maintain a stable idle. Carbon deposits from crankcase vapors can coat the pintle and seat, preventing it from fully opening or closing. Cleaning with IAC-safe cleaner often resolves P0505 without replacement. If the motor winding has failed, the valve won't respond to PCM commands at all.
🔩 Part
$30–$120
👨🔧 Labor
$50–$100
⚡ DIY Difficulty
Easy
25%
#2 - Check First
Carbon Buildup in Throttle Body
Heavy carbon deposits on the throttle plate and bore walls restrict the bypass air channel used by the IAC system, making it impossible for the PCM to achieve its idle target. Throttle body cleaning - using throttle-body cleaner and a rag while the engine is off - often cures P0505 when the IAC valve itself is still functional.
🔩 Part
$8–$15
👨🔧 Labor
$50–$80
⚡ DIY Difficulty
Easy
12%
#3 - Less Common
Vacuum Leak
An unmetered air leak downstream of the MAF sensor introduces extra air into the intake that the IAC system can't compensate for. Common sources include cracked intake boots, disconnected PCV hoses, or failed intake manifold gaskets. The PCM sees an idle that won't stabilize and logs P0505. Use a smoke machine or carb cleaner to locate the leak.
🔩 Part
$5–$60
👨🔧 Labor
$60–$150
⚡ DIY Difficulty
Medium
8%
#4 -
PCM Idle Control Fault
In rare cases the PCM's idle control logic or driver circuit fails, preventing proper IAC commands from being sent. This is a diagnosis of exclusion - confirm the IAC, throttle body, and vacuum integrity are all acceptable before suspecting the PCM. PCM replacement requires programming to the vehicle's VIN.
🔩 Part
$200–$800
👨🔧 Labor
$100–$200
⚡ DIY Difficulty
Hard
🚗 Most Affected Vehicles
🔧 Step-by-Step Diagnosis
- Clean the IAC Valve and Throttle Body - Remove the IAC valve and spray it with mass-airflow-safe cleaner. Wipe the throttle plate and bore free of carbon deposits with a lint-free cloth. Reinstall, clear the code, and check if it returns. This resolves over half of P0505 cases at zero parts cost.
- Inspect for Vacuum Leaks - With the engine at idle, listen and feel for hissing air near the intake manifold, PCV hose, brake booster line, and throttle body boot. A can of carb cleaner sprayed at suspected areas (engine running) will cause an RPM change if a leak is present. Identify and seal any leaks found.
Watch: P0505 Diagnosis
Visual walkthrough - IAC cleaning, throttle body service, and vacuum leak testing for P0505.
📍 Find a Trusted Shop Near You
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Tips for Choosing a Shop
- Ask if they charge a diagnostic fee and whether it applies toward the repair
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- Check Google reviews for recent mentions of the specific repair you need