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.

Codes › P0505

P0505: The idle speed control system is not working properly

Idle Air Control System Malfunction

Moderate severityCan you keep driving?

Drive to a shop soon. The car may stall at idle, especially when slowing down or sitting in traffic. Erratic idle is annoying and can become a safety issue at low-speed stops.

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 it costs

IAC valve cleaning$50 to $100 at a shop
Throttle body cleaning$80 to $150
IAC valve replacement$100 to $300

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 the live idle speed and the commanded IAC position or throttle idle target, so you can see whether the system is commanding the right value and the engine is simply not responding. Download Lewis.

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.