P0505: Idle Control System Fault
P0505 means the engine computer found that the engine idle speed is not staying where it should, and it is a general idle control fault. It is usually not an emergency, but a stalling engine can become a safety problem, so have it checked soon.
You can drive, but get it checked soon
The engine often keeps running, and limp mode is not typical for this code. But if it stalls at idle or while coasting, you can lose power steering assist and brake booster help at a stop or in traffic, so keep trips short until it is fixed.
What it means
The engine computer sets a target idle speed and watches whether the engine reaches it. For gasoline engines, the rule is that a fault must be flagged if idle misses the target by more than 200 rpm too high or 100 rpm too low (diesels: more than 30 %). Older engines use a separate idle air control valve that lets a small amount of air around the closed throttle. Engines with an electronic throttle have no such valve, and the throttle itself controls idle. P0505 is the general code used when the manufacturer does not separate too low from too high, or when the control has run out of adjustment range. The check engine light is expected to come on, though lamp behavior depends on the manufacturer. On some Ford engines from 2020 to 2023, this code also turns off start-stop.
Likely causes
- Throttle body or idle valve dirty or gummedVery likely
Deposits restrict or stick the air path that controls idle, so the computer cannot hold the target speed.
- False air (hoses, intake gasket, crankcase ventilation)Possible
Air that enters the intake without being measured upsets the idle mixture and speed.
- Idle valve faulty mechanically or electricallyPossible
A valve that sticks or has an electrical fault cannot adjust the idle air as commanded.
- Learned values not reset after cleaningLess likely
After the throttle body is cleaned, the computer may still use old stored idle settings until they are relearned.
What you may notice
- Idle speed fluctuates, too high or too low
- Engine stalls at idle or when coasting
- Start-stop not working
- Check engine light on
What to check, cheapest first
- 1Look for a loose, cracked or disconnected vacuum hose or crankcase ventilation hose, and listen for a hissing sound at idle. · you can do it
- 2Inspect the throttle body for black gummy deposits around the throttle plate and clean it with a cleaner made for throttle bodies, following the vehicle's service instructions. · you can do it
- 3After cleaning, clear the code and let the computer relearn idle, using the idle relearn procedure for your vehicle if one exists. · you can do it
- 4Read the stored code and any related codes (such as P0506 or P0507) with a scan tool and note the freeze frame data. · mechanic
- 5Check for false air at the intake gasket, for example with a smoke test. · mechanic
- 6On engines with an idle valve, test the valve's movement and electrical circuit, and replace it if it fails. · mechanic
Related codes
Frequently asked questions
Can I drive with P0505?+
Usually yes for short trips, but the engine may stall at idle or when coasting. Get it checked soon.
Can a dirty throttle body cause P0505?+
Yes. It is the most likely cause. Deposits restrict the air that controls idle, and cleaning often fixes it.
Will P0505 pass an emissions test?+
Probably not. The code is emissions relevant and turns on the check engine light. After the repair, a drive cycle (a set pattern of driving) is needed before the monitor reruns.
Do I need to reset anything after cleaning the throttle body?+
Possibly. If the computer keeps old learned idle values, the fault can return, so clear the code and have the idle relearned.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10