P0518Generic code

P0518 Code: Idle Air Control System Circuit Intermittent

The P0518 code means the computer sees intermittent dropouts in the idle air control valve circuit. It can cause a rough or high idle, or an engine that stalls. Because the fault comes and goes, you have to reproduce it to find it.

Severity
Medium
Can I keep driving?
Yes, but with caution
Estimated cost
$40 to $350
Difficulty
Medium
System
Idle and Speed Control
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0518 code means

On engines with a throttle cable, the idle air control (IAC) valve lets air bypass the throttle plate so the engine holds a steady idle when loads come on (air conditioning, power steering, lights) or when the engine is cold. On electronic throttles, the throttle body itself does that job.

The computer watches the control circuit from its own driver transistor. The P0518 code is set when it sees intermittent dropouts in that circuit. Before you replace the part, check the fuse, relay, connector and wiring, because those are the most common causes.

Carbon buildup is enemy number one here. It clogs the valve and the air passages, and a lot of the time the problem goes away after you clean the valve and the throttle body with the right cleaner. After that, an idle relearn procedure is usually needed.

Intermittent faults are found by reproducing them. With the engine running and the scan tool in graph mode, move the wiring harness section by section, tap the connector gently and, if needed, warm the area with a hair dryer or wet the connector with a spray bottle. The spot where the reading jumps is where the fault is.

P0518 code symptoms

  • Check engine light or another warning light on.
  • Rough idle, high idle, or an engine that stalls.
  • Idle that hunts, rising and falling on its own.
  • The engine stalls when you come to a stop or when you turn on the air conditioning.
  • Very high idle speed.

Likely causes of the P0518 code

  1. Poor contact. Loose or corroded pins.
  2. Wire broken inside. The insulation looks intact.
  3. Relay with worn contacts that sometimes does not close.

How to diagnose the P0518 code step by step

  1. Command the component with the scan tool and check whether it responds.
  2. Check the fuse and relay. Measure voltage on both sides of the fuse, and swap the relay with an identical one.
  3. Measure the supply at the connector. You should see battery voltage with the key on or when the component is commanded.
  4. Measure the component resistance and compare it with the manual. Also check its insulation to ground.
  5. Check the control wire for continuity all the way to the computer and for any short.
  6. Clean the valve and the throttle body and repeat the test.
  7. Perform the idle relearn according to the manufacturer.

How to fix the P0518 code

  • Repair the connector, the wiring or the supply.
  • Replace the idle air control valve if it does not respond when it gets a correct supply and command.
  • Clear the codes and verify the repair.

Technical reference data

  • Stepper motor valve (4 or 6 wires) or rotary valve (2 or 3 wires).
  • Typical hot idle speed: 650 to 850 rpm.
  • These values are a general guide: check the manufacturer data for your vehicle.

Brand-specific notes

This code is very common on cable-throttle engines. When it comes and goes, suspect the connector and the harness before the valve.

P0518 on Honda

Very common on Honda engines with a throttle cable.

P0518 on Nissan

Very common on Nissan engines with a throttle cable.

P0518 on GM

Very common on GM engines with a throttle cable.

P0518 on Chrysler

Very common on Chrysler engines with a throttle cable.

Frequently asked questions about P0518

What does the P0518 code mean?
It is a generic OBD2 code that means the computer sees intermittent dropouts in the idle air control valve circuit. The fault comes and goes, which usually points to a poor contact or a broken wire.
Can I drive with a P0518 code?
Yes, but with caution. A rough idle or a stall at a stop sign can be a hazard in traffic, so get it checked soon.
What does the idle air control valve do?
On engines with a throttle cable, the IAC valve lets air bypass the throttle plate so the engine holds a steady idle when loads come on (air conditioning, power steering, lights) or when the engine is cold. On electronic throttles, the throttle body itself does that job.
Do I always have to replace the part?
No. Check the fuses, relays, connectors and wiring first. For an intermittent fault, those are the likeliest culprits.
How much does it cost to fix P0518?
As a rough guide, between $40 and $350. A connector repair is at the low end, and a new valve with labor is at the high end.
Will a P0518 code clear itself?
The light may go out if the fault stays away for several drive cycles, but an intermittent P0518 code usually returns. Find the poor contact instead of waiting for it.
What is the most common cause of P0518?
A poor electrical contact, such as loose or corroded pins in the connector. A wire that is broken inside its insulation and a worn relay come next.

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