What causes fault code P0518?

• Most likely: Damaged wiring or loose connections at the IAC valve connector • Likely: Faulty Idle Air Control (IAC) valve with internal electrical failure • Possible: Excessive carbon buildup causing the valve to stick mechanically • Rare: Powertrain Control Module (PCM) internal fault or software issue

How do you fix fault code P0518?

Visual inspection: Check the IAC valve connector for corrosion, debris, or backed-out pins. Clean with contact cleaner. Measurement: Use a multimeter to check the internal resistance of the IAC valve. Typical values are 10-25 Ohms (consult service manual). Verify reference voltage (5V or 12V) from the PCM. Cleaning: Remove the IAC valve and clean carbon deposits using throttle body cleaner. Ensure the valve moves freely. Advanced diagnosis: Use a scan tool to perform an 'Active Test' on the IAC valve to see if the idle RPM fluctuates as commanded. Replacement: If the valve shows incorrect resistance or fails to respond despite good wiring, replace the IAC valve.

Powertrain (Engine & Gearbox)Standard EOBD

P0518 - Idle Air Control Circuit Intermittent

Q: What are the symptoms of fault code P0518?

• Check engine light on (MIL) • Erratic or fluctuating idle speed • Engine stalling at idle (e.g., at stop lights) • Difficulty starting the engine without pressing the gas pedal • Performance issues during low-speed maneuvers

Last updated:1 February 2026

Related:Idle Air Control (IAC) Valve, Throttle Body, PCM/ECM, Wiring Harness

System

Powertrain (Engine & Gearbox)

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Technical Description

P0518 is a generic OBD-II diagnostic trouble code indicating that the Powertrain Control Module (PCM/ECM) has detected an intermittent electrical signal from the Idle Air Control (IAC) valve or its circuit. The IAC valve regulates the amount of air bypassing the throttle plate to maintain a steady idle speed. The PCM monitors the duty cycle or voltage in the circuit; if the signal fluctuates outside predefined parameters unexpectedly, P0518 is stored. This is typically caused by wiring harness issues or internal sticking within the IAC assembly affecting electrical resistance.

Possible Causes

Damaged wiring or loose connections at the IAC valve connector

Faulty Idle Air Control (IAC) valve with internal electrical failure

Excessive carbon buildup causing the valve to stick mechanically

Powertrain Control Module (PCM) internal fault or software issue

Symptoms

Check engine light on (MIL)

Erratic or fluctuating idle speed

Engine stalling at idle (e.g., at stop lights)

Difficulty starting the engine without pressing the gas pedal

Performance issues during low-speed maneuvers

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P0518 - Idle Air Control Circuit Intermittent

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P0518 - Idle Air Control Circuit Intermittent

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