The most common causes, most likely first: Carbon buildup in the throttle body or IAC passage; Failed idle air control valve; Vacuum leak; Dirty or failing throttle position sensor; Throttle body relearn not performed.

P0505 Idle Air Control System Malfunction

The computer can't hold a steady idle — usually because the throttle body and idle passages are caked with carbon.

Electrical & idle

At idle the engine needs a small, precisely controlled amount of air. Older engines meter it through a dedicated idle air control valve; newer ones move the electronic throttle plate slightly. P0505 means the ECU commanded an idle speed and couldn't achieve it. On most vehicles the cause is deposits rather than a failed part, which makes cleaning the first and often the only step needed.

Diagnose P0505 on your specific vehicle Add your year, make and model for ranked causes, repair videos and the exact parts you need

Can I keep driving?

Driveable

Yes, but stalling at stops makes it unpleasant and mildly hazardous.

How serious

Moderate. Stalling at junctions is a genuine safety nuisance, especially with power assist dropping out.

Symptoms you'll notice

Idle speed too high or too low
Hunting or surging idle that won't settle
Stalling when coming to a stop or when the air conditioning engages
Hard starting
Check-engine light

What causes P0505

Ordered by what to check first — how likely each cause is, weighed against what it costs to rule out. Working down the list in order is what stops you replacing an expensive part to fix a cheap problem.

Carbon buildup in the throttle body or IAC passage

The dominant cause. Oil vapour from the PCV system condenses in the throttle body and bakes into a hard deposit that narrows the idle air passage. Cleaning it with throttle body cleaner and a rag fixes this a large fraction of the time, for the price of a can of cleaner.

Failed idle air control valve

On vehicles that have a separate IAC valve, it can seize or fail electrically. Clean it before replacing it.

Vacuum leak

Unmetered air entering elsewhere means the ECU can't control idle through its own valve. Often stored alongside P0171.

Dirty or failing throttle position sensor

The ECU needs to know where the throttle plate actually is to control idle.

Throttle body relearn not performed

On drive-by-wire vehicles, cleaning or replacing the throttle body requires an idle relearn procedure afterwards. Skipping it sets this code on a perfectly clean throttle body — a genuinely common self-inflicted cause.

How to diagnose it

Remove the intake duct and inspect the throttle body bore and plate for carbon deposits.
Clean the throttle body and, if fitted, the IAC valve and its passage with throttle body cleaner.
Perform the idle relearn procedure afterwards if the vehicle needs one — otherwise the code returns.
Check for vacuum leaks and any lean codes stored alongside.
Test the IAC valve's resistance against spec on vehicles that have one.
Compare commanded against actual idle speed on live data.

Parts you might need

Tools for the job

Throttle body cleaner
Screwdriver set
OBD-II scanner with live data
Torque wrench

Related trouble codes

Get the parts list for your vehicle Ranked causes, tutorials and live part prices for your exact year, make and model

Fitment matters. Causes and parts above are the common cases for P0505 across vehicles. The specific part numbers, procedures and torque specs vary by year, make, model and engine — always confirm against service data for your vehicle before buying parts or starting work.