P0508Generic code

P0508 Code: Idle Air Control System Circuit Low

The P0508 code means the computer sees low voltage in the control circuit of the idle air control valve, either a short to ground or a missing supply. It can cause a rough or high idle, or an engine that stalls. Check the fuse, relay, connector and wiring before you replace the valve.

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

On this page

What the P0508 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 P0508 code is set when it sees low voltage in that control circuit, which means a short to ground or a missing supply. 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.

To confirm the fault, unplug the component with the key on and watch what happens to the reading or the code. If the value jumps to the opposite extreme, the wiring is fine and the component is the one pulling the signal to ground. If nothing changes, look for the short in the wire, starting where the harness rubs against the engine, the chassis or the exhaust.

P0508 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 P0508 code

  1. Control wire shorted to ground. The component can end up switched on all the time, or the computer shuts off the command to protect itself.
  2. Interrupted supply. A blown fuse, a bad relay or a broken wire.
  3. Shorted component. Its resistance is far lower than specified.
  4. Connector with moisture. It creates a path to ground.

How to diagnose the P0508 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 P0508 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, and the usual cure is a good cleaning plus an idle relearn.

P0508 on Honda

Very common on Honda engines with a throttle cable.

P0508 on Nissan

Very common on Nissan engines with a throttle cable.

P0508 on GM

Very common on GM engines with a throttle cable.

P0508 on Chrysler

Very common on Chrysler engines with a throttle cable.

Frequently asked questions about P0508

What does the P0508 code mean?
It is a generic OBD2 code that means the computer sees low voltage in the control circuit of the idle air control valve. That points to a short to ground or a missing supply.
Can I drive with a P0508 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. Carbon buildup is also very common, and cleaning often solves it.
How much does it cost to fix P0508?
As a rough guide, between $40 and $350. A cleaning or a connector repair sits at the low end, and a new valve with labor is at the high end.
Will a P0508 code clear itself?
Only if the fault was temporary, such as moisture in a connector. If the short or the open supply is still there, the P0508 code comes back and the check engine light turns on again.
What is the most common cause of P0508?
A control wire shorted to ground or a supply problem such as a blown fuse or a bad relay. Moisture in the connector and a shorted valve are also common.

Related codes

Questions about the P0508 code

There are no questions about this code yet. Tell us about your case: vehicle, symptoms and what you have already checked.