P0505: Idle Air Control System Malfunction (Meaning, Causes, Fixes)
What P0505 means
At idle the throttle plate is closed, so the idle air control valve or motor lets a metered amount of air bypass it to set the idle speed. The powertrain control module (PCM) opens or closes it to keep idle steady as loads like the A/C, heater and wipers change. The sources describe the trigger differently, and both are worth knowing. OBD-Codes.com says P0505 sets when the PCM sees an incorrect voltage on one or more of the IAC motor circuits, which points to an electrical fault. RepairPal says it sets when the PCM cannot achieve the desired idle RPM, which also covers mechanical causes like vacuum leaks and carbon buildup. OBD-Codes.com also notes that IAC problems only show up at idle or when the throttle is closed.
Severity: can I keep driving?
Drive with caution, fix soon
Usually drivable with care, but get it fixed soon. Both sources say the engine may idle too low and stall at stops or when you let off the throttle, which can leave you without power steering and power brake assist in traffic. If it stalls, keep a bit of throttle on at stops only if it's safe, and get it looked at before driving in heavy traffic.
Common symptoms
- Check engine light on (both sources)
- Engine stalls or idles very low, especially at stops or when letting off the throttle (both sources)
- Engine may not idle unless the throttle is pressed (OBD-Codes.com)
- Rough idle (RepairPal)
- Idle speed too high in some cases (both sources)
Likely causes
Roughly most to least common, based on the sources below. Your vehicle may differ.
- Failed idle air control motor or valve (both sources)
- Damaged IAC connector or pins, including moisture in the connector (OBD-Codes.com)
- IAC circuits open, shorted to ground or shorted to voltage (OBD-Codes.com)
- Intake manifold vacuum leaks (RepairPal)
- Carbon buildup in the throttle body air passages (RepairPal)
- Failed PCM, which is less common (OBD-Codes.com)
Diagnostic steps you can do
Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.
- Check the IAC connector and pins for damage, corrosion or moisture, and look over the wiring for chafed or melted spots.
- Listen and look for vacuum leaks, such as cracked or disconnected hoses around the intake manifold, since RepairPal lists them as a common cause.
- Inspect the throttle body and its idle air passages for carbon buildup. RepairPal's most common misdiagnosis is replacing the IAC motor when carbon buildup was the real problem.
- If you're comfortable with electrical testing, OBD-Codes.com describes a test: clear the code, unplug the IAC and start the engine. If P0505 doesn't come back, the IAC is likely shorted internally; if it does come back, suspect the wiring or PCM. If you're not sure, leave this to a shop.
Typical fixes
- Clean carbon from the throttle body and idle air passages
- Repair vacuum leaks
- Repair damaged IAC wiring or connectors
- Replace a failed idle air control motor or valve
- Replace or repair the PCM only after wiring and the IAC are ruled out
Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.
🛠 When to call a pro
- RepairPal warns that the IAC motor is often replaced when carbon buildup was the cause, so ask the shop whether they checked and cleaned the throttle body first.
- OBD-Codes.com's wiring checks involve unplugging components and isolating circuits at the PCM connector; a shop can do this without risking damage.
- On cars with an electronic throttle body and no separate IAC valve, idle is controlled by the throttle body itself, so diagnosis follows the manufacturer's procedure for that system.
Where to buy
Plain search links. Always match parts to your exact year, make, model and engine.
FAQs
Can P0505 make my car stall?
Yes. Both OBD-Codes.com and RepairPal list stalling, especially at stops or when you let off the throttle, as a P0505 symptom.
Will cleaning the throttle body fix P0505?
Sometimes. RepairPal lists carbon buildup in the throttle body air passages as a common cause and says the IAC motor is often replaced when cleaning was the real fix. If the fault is electrical, as OBD-Codes.com describes, cleaning won't help.
Is P0505 an electrical problem or an idle problem?
The sources differ. OBD-Codes.com describes it as an incorrect voltage on the IAC circuits, while RepairPal describes it as the computer being unable to reach the target idle speed. In practice, check both the wiring and mechanical causes like vacuum leaks and carbon.
Sources
Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.