The most common causes are: Carbon buildup in the throttle body or IAC passage (very common); Failed IAC valve; Vacuum leak; Dirty or failing throttle position sensor; Failing PCV valve.

How much does it cost to fix P0505?

Typical repairs range from $80–$250 for clean throttle body / iac passage up to $300–$900 for replace throttle body. Costs vary by vehicle and region.

P0505powertrain

P0505 – Idle Control System Malfunction

What does P0505 mean?

The idle air control (IAC) system is not maintaining the commanded idle speed. On older vehicles this involves a dedicated IAC valve; on newer drive-by-wire vehicles the throttle body itself controls idle. Either way, the computer has detected that actual idle speed does not match what it commanded.

What causes P0505?

Listed from most to least common:

1Carbon buildup in the throttle body or IAC passage (very common)
2Failed IAC valve
3Vacuum leak
4Dirty or failing throttle position sensor
5Failing PCV valve
6Throttle body needing relearn after cleaning or battery disconnection

Code breakdown

SystemP — Powertrain

TypeGeneric (standardized SAE / OBD-II code — same meaning across most manufacturers)

SubsystemVehicle speed control, idle control, and auxiliary inputs

Common symptoms

Check engine light on
Idle speed too high, too low, or fluctuating
Engine stalls at stops or when coming to a halt
Rough idle
Engine may need throttle input to stay running when cold

How to diagnose

1Remove and clean the throttle body — carbon buildup around the throttle plate is the most frequent cause.
2On vehicles with a separate IAC valve, remove and clean it, then test its operation.
3Perform an idle relearn procedure after cleaning. Skipping this step causes the code to return on many vehicles.
4Smoke-test for vacuum leaks.
5Check the PCV valve for correct operation.
6Verify the throttle position sensor reads smoothly through its range.

Can cause stalling, which is a safety concern in traffic. Worth addressing promptly if the engine is actually stalling.

Common fixes and estimated costs

Repair	Estimated cost
Clean throttle body / IAC passage	$80–$250
Replace IAC valve	$150–$400
Repair vacuum leak	$50–$400
Replace throttle body	$300–$900

Costs are rough US estimates including parts and labor. Actual pricing varies significantly by vehicle, region, and shop rates.

Good to know

The idle relearn step is frequently skipped and is a common reason the code returns after cleaning. Procedures vary by manufacturer — look up the specific relearn sequence for your vehicle.

Back to all Powertrain Codes →

This information is provided for general reference only and may vary by manufacturer, model, and year. It is not a substitute for professional diagnosis or repair.