P0518: Idle Air Control Circuit Intermittent

What it means

The idle air control circuit signal is intermittent or erratic. The fault may be in the circuit or its connections.

At a glance

• Usually lights the check-engine light

How it works

The idle air control circuit operates the valve that adjusts bypass airflow while the throttle is closed. P0518 sets when the control module detects an intermittent or erratic electrical signal in that circuit, rather than a steady open or short.

Common causes

Most likely first.

1. Intermittent contact at connector
2. Connector with intermittent contact under vibration
3. Harness routed against vibrating component

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. Check freeze-frame data and live idle speed and IAC command to see when the intermittent fault coincides with an idle change.
2. Inspect the IAC connector and harness routing for loose terminals, chafing, or contact with a vibrating or moving part.
3. With the engine idling, monitor the circuit or scan-tool status while gently moving the harness near the valve to look for a signal dropout.
4. Check connector terminal tension and test the IAC wires for intermittent continuity or shorts while flexing suspect sections.
5. If the wiring remains stable, test the IAC actuator using the manufacturer's procedure.

Common fixes

• Secure a loose IAC connector and restore proper terminal contact.
• Repair chafed or poorly routed wiring that moves against a vibrating component.
• Replace damaged connector terminals or seals that allow intermittent contact.
• Replace the IAC valve or actuator if the circuit remains intermittent at the component.

Can I keep driving?

A short trip may be possible if idle remains stable, but intermittent circuit faults can cause sudden stalling at stops or after a cold start. Avoid heavy traffic and stop if the engine begins to stall or idle unpredictably.