P0518

What it means

P0518 is the generic code for an intermittent fault in the idle air control (IAC) circuit. The engine control unit monitors the circuit that drives the idle speed actuator and sets the code when it sees a signal or current that drops out, flickers or goes out of range from time to time rather than staying faulty. Layout and strategy vary by engine, so some applications use a stepper motor, others a rotary or solenoid valve, or an electronic throttle body doing the same job.

Symptoms

• Idle speed that hunts, surges or wanders up and down
• Rough or unstable idle that comes and goes
• Engine stalling at idle or when coming to a stop
• Engine stalling or dipping when the air conditioning or electrical loads switch on
• Hard starting or slow to settle after a cold start
• Malfunction indicator lamp on, sometimes going out again

Possible causes

• Loose, corroded or damaged connector at the idle air control valvelikely
• Chafed, broken or high-resistance wiring between the control unit and the idle actuator, including intermittent breaks that show up with engine movement or vibrationlikely
• Worn or failing idle air control valve with an intermittent internal winding fault or a sticking mechanismlikely
• Carbon build-up in the throttle body or idle air passage causing the valve to struggle or draw irregular currentpossible
• Poor power or earth supply to the actuator, such as a weak earth point or a poor relay or fuse contactpossible
• Vacuum leak or unmetered air upsetting idle control and making the valve work outside its normal rangepossible
• Internal fault in the engine control unit driver circuitrare

How to diagnose

1. Read all stored and pending codes and the freeze frame data, noting engine temperature, speed and load when the fault set, then check for related codes such as idle speed or throttle faults
2. Visually inspect the idle air control valve or throttle body connector and nearby wiring for corrosion, bent pins, looseness, oil contamination and chafing
3. With the engine idling, gently wiggle the connector and harness and watch for idle changes or the fault returning
4. Check the throttle body and idle air passages for carbon or deposits, and check that the valve moves freely, following the procedure for the specific engine
5. Check the actuator's power, earth and control signals against the manufacturer's wiring diagram, and measure the winding resistance against specification
6. Look for vacuum leaks and air intake leaks, checking hoses, the intake manifold gasket and the brake servo hose
7. Check battery condition and charging voltage, and the earth connections for the engine and control unit
8. Back-probe or use a scope on the control circuit while the fault is present to catch dropouts, if the cause is still not found

How to fix

• Clean, repair or replace the damaged connector terminals and secure the connector properly
• Repair or replace damaged wiring sections, using proper soldered and sealed joins, and secure the harness away from heat and rubbing
• Replace the idle air control valve or throttle body assembly if it fails electrical or movement checks, and carry out any required idle relearn or adaptation afterwards
• Clean the throttle body and idle air passages with a suitable cleaner as the manufacturer recommends
• Restore good power and earth supplies by cleaning earth points and replacing faulty fuses or relays
• Repair any vacuum or intake leaks, then clear the codes and confirm that the idle remains stable on a test drive

Can I keep driving?

Nearby codes

• P0517GenericBattery Temperature Sensor Circuit High
• P0519GenericIdle Air Control System Performance