P0505

What P0505 means

This code means the computer noticed your idle speed isn't where it expects it to be. The engine idles too high, too low, or hunts up and down when it should be steady. It's not naming a broken part. It's saying the system that manages idle airflow isn't behaving the way it's supposed to, and something in that system needs a look.

Is it safe to drive

Usually yes, for now. A rough or unstable idle is annoying more than dangerous. Watch for the engine stalling at stops, especially in gear when you're braking or turning, since a stall in traffic is the real risk here. Also watch for the idle racing high on its own, which can catch you off guard when you shift into reverse or drive. If either of those is happening, don't wait on this.

What commonly causes it

On older engines with a dedicated idle air control (IAC) valve, that valve itself is a common cause, usually from carbon buildup gumming up the moving parts rather than the valve failing outright. On newer engines that control idle through the electronic throttle body instead of a separate IAC valve, the throttle body and its position sensors are more often the issue.

Beyond that part, the usual suspects include:

A vacuum leak, which lets in extra unmetered air the computer can't account for. A dirty throttle body, where carbon buildup on the throttle plate changes how much air gets through at idle. A carbon-fouled IAC passage, even when the valve itself is fine. Wiring or connector problems at the IAC valve or throttle body. Less often, a bad idle-related sensor, or a PCM that needs a relearn or reset after a repair or battery disconnect.

Which of these is likely depends a good deal on the engine. A car with a separate IAC valve points diagnosis one direction, a car running full electronic throttle control points another.

What diagnosis and repair involve

Diagnosis starts with checking for vacuum leaks and inspecting the throttle body and IAC valve for carbon buildup, then watching live data on a scan tool to see how idle airflow behaves compared to what the computer commands. That's usually 30 to 60 minutes of shop time before we know what we're actually dealing with.

If it turns out to be a carbon buildup problem, cleaning the throttle body and IAC passage is a relatively inexpensive job, often in the range of $150 to $250 including labor. If the IAC valve or throttle body actually needs replacing, parts alone can run higher, and total cost varies a lot by make and model. A vacuum leak can be anywhere from a quick hose fix to something more involved depending on where it is.

We wouldn't recommend just buying an IAC valve or throttle body off the shelf and swapping it in. The code doesn't confirm which part failed, and diagnosis first is what keeps you from paying for a part that wasn't the problem.

Generic OBD-II guidance — manufacturer-specific codes and causes vary by make, so always confirm against your vehicle. A code identifies a system or condition; it does not, on its own, prove which part has failed.