P0505: The idle speed control system is not working properly
Idle Air Control System Malfunction
The idle control system cannot hold a steady idle, so the car may stall when you slow down or stop. Get it checked within a few days. If it stalls in traffic, have it towed.
Stop driving and get help if
- The check engine light is flashing
- The temperature gauge runs high, or you see steam (overheating)
- The car loses power, hesitates badly or stalls
- You smell burning or see smoke
- A brake warning light comes on, or the brakes feel different
Pull over as soon as you can, turn the engine off, and call a tow or roadside help.
What it means
Older fuel-injected engines use an idle air control (IAC) valve to let air bypass the throttle plate and keep idle speed steady regardless of load from the AC, alternator, or power steering. P0505 means the computer cannot control idle speed within its target range. The IAC valve is an often-overlooked source of rough idle, hunting idle, and stall-on-deceleration complaints. Before replacing the valve, try cleaning it. Carbon deposits on the pintle and bore are the most common cause of IAC problems and they respond well to throttle body cleaner.
Common causes
- Carbon-clogged IAC valveCarbon deposits build up on the pintle and bore over time, restricting movement. Cleaning often fixes it.
- Failed IAC valve motor or solenoidThe electrical actuator inside the valve fails and cannot position the pintle.
- Vacuum leak in the intakeUnmetered air bypassing the IAC confuses idle control and can set P0505.
- Dirty throttle bodyCarbon buildup on the throttle plate and bore affects airflow even when the IAC is working.
- Faulty IAC wiring or connectorBroken wires or corroded pins prevent the PCM from commanding the valve.
What the fix could involve
From the cheaper fix to the bigger one. Prices depend on your car, your area and the shop, so ask for a written quote that names the part and the labor before you approve it.
- IAC valve cleaning
- Throttle body cleaning
- IAC valve replacement
Symptoms
- Rough or unstable idle speed
- Engine stalls when coming to a stop
- High idle that does not come down after warmup
- Idle speed fluctuates up and down
- Hesitation when coming off the throttle
What to do next
Note that many newer vehicles (roughly 2005 and later) do not have a separate IAC valve. They use electronic throttle control instead, which means P0505 on a newer vehicle points toward the throttle body motor or pedal sensor. On older vehicles, remove and clean the IAC valve first before replacing it.
Lewis shows live idle RPM, throttle position, and engine load together so you can see whether the idle is hunting or stuck. Get Lewis on the App Store or Google Play.
Related codes
FAQ
- Can a dirty throttle body cause P0505?
- Yes. Heavy carbon on the throttle bore restricts airflow even when the IAC is working correctly. Cleaning both at the same time is good practice.
- My car stalls at every red light. Is this the problem?
- P0505 is a strong candidate, but also check for vacuum leaks and a dirty throttle body. All three can produce that exact symptom.
- Do newer cars have this problem?
- Newer cars with drive-by-wire do not use a separate IAC valve. They use the throttle body motor for idle control. P0505 on these vehicles often points to a throttle body or APP sensor issue.
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