Idle Control System Malfunction
The P0505 code means the vehicle's onboard diagnostic system has detected: Idle Control System Malfunction. This is a moderate severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- medium
- Estimated cost
- $30-$80 for IAC valve; $0 for cleaning
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Common Symptoms
- Rough idle
- Idle too high or too low
- Engine stalling at idle
- Idle fluctuation between 400 and 1500 RPM
Probable Causes (Ranked by Likelihood)
- Dirty or failed idle air control (IAC) valve Very High
- Vacuum leak causing an uncontrolled idle High
- Dirty throttle body with carbon buildup High
- Failed throttle position sensor Moderate
Step-by-Step Diagnostic Procedure
-
Remove the IAC valve and clean its pintle and housing with throttle body cleaner. Carbon buildup on the valve prevents proper idle control.
-
Inspect all vacuum hoses for cracks or disconnection.
-
Clean the throttle body: carbon buildup around the throttle plate causes erratic idle, especially on direct-injection engines.
-
Monitor IAC counts on a scan tool. Excessively high counts (above 80-100) indicate the IAC is working overtime to compensate for a vacuum leak.
Common Fixes by Vehicle
What techs usually find when diagnosing P0505 on specific vehicles — tap your vehicle for the fix and the exact part:
1999-2012 Honda Accord/Civic 2.4L easy DIY
The IACV (idle air control valve) gets clogged with carbon deposits. Remove it (2 bolts on the throttle body) and clean with throttle body cleaner. Honda part 16022-RAA-A01. If cleaning does not fix the surging idle, replace the IACV. Also clean the throttle body bore while the IACV is off.
Labor: 30 min2002-2015 Nissan Altima/Sentra 2.5L QR25DE easy DIY
Clean the IACV and throttle body together. Nissan QR25DE engines accumulate heavy carbon in the throttle body. Remove the IACV (Hitachi ABV0035) and soak in carb cleaner for 30 minutes. Also check for vacuum leaks at the intake manifold gaskets. Perform an idle relearn after cleaning (turn key on, wait 10 seconds, start engine, let idle 10 minutes).
Labor: 30-45 min2000-2013 Toyota Corolla/Matrix 1.8L easy DIY
The IACV on Toyota 1ZZ-FE engines clogs around 100K. Remove and clean with throttle body cleaner. If the idle surges between 500-1500 RPM, also check for a vacuum leak at the brake booster hose. After cleaning, perform the idle relearn: warm engine, turn off AC and lights, let idle for 10 minutes with trans in park.
Labor: 30 min2004-2013 Chevrolet Silverado/Tahoe 5.3L easy DIY
The IAC valve (ACDelco 217-435) is located on the throttle body. Clean it first with throttle body cleaner -- often fixes the issue. If the idle hunts after cleaning, the IAC pintle may be worn and needs replacement. Also check the throttle position sensor (TPS) for erratic voltage. GM TSB #08-06-04-022 addresses idle concerns.
Labor: 30 minFrequently Asked Questions
Can cleaning the IAC valve fix P0505?
Often yes. Carbon deposits prevent the IAC valve pintle from moving freely, causing inconsistent idle control. Cleaning with throttle body cleaner is the first step and may resolve the code without replacing the valve.
Sources
This page is built from documented references. Verify against your own service info before repair work.
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