P0505: Idle Control System

What it means

A general fault has been detected in the idle air control (IAC) system. On older vehicles with a separate IAC valve, this means the valve cannot maintain commanded idle RPM; on drive-by-wire systems, the equivalent fault is in the throttle body's idle adaptation.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The idle control system regulates airflow so the engine can maintain a stable speed when the accelerator is released. P0505 sets when the module detects that idle control is not responding or cannot maintain the expected idle, through a separate idle air control valve on some vehicles or electronic throttle control on others.

Common causes

Most likely first.

1. IAC valve carboned, sticky
2. Throttle body dirty (drive-by-wire)
3. Vacuum leak affecting idle

Symptoms

• Check engine light on
• Idle speed too high, too low, or unstable
• Possible stalling at idle or after cold start

How to diagnose it

1. Read freeze-frame data and observe commanded and actual idle speed to identify when the control problem occurs.
2. Inspect the idle air control valve or throttle body, as fitted, along with connectors and wiring for damage or poor connections.
3. Check intake and vacuum hoses for leaks that can raise or destabilize idle speed.
4. Inspect the idle air passage or throttle body for carbon buildup and clean it only according to the manufacturer's procedure.
5. Test the idle air control valve or electronic throttle operation and adaptation using the manufacturer's diagnostic procedure.

Common fixes

• Repair a leaking intake or vacuum hose affecting idle.
• Clean a carboned idle air passage or throttle body according to the manufacturer's procedure.
• Repair a damaged idle control connector or wiring.
• Replace a sticking idle air control valve if testing confirms it has failed.
• Perform the required throttle idle adaptation if the manufacturer's procedure calls for it.

Can I keep driving?

If the engine stalls in traffic or cannot hold a safe idle, avoid driving until the fault is corrected. If it remains stable, drive cautiously and avoid situations where a sudden stall would be hazardous.