P0508 Code: Idle Air Control System Circuit Low
The idle air control circuit reads low voltage – a shorted valve coil, lost power feed or wire to ground.
At a glance
Severity: Low
- Meaning
- Idle Air Control System Circuit Low
- System
- Speed & Idle Control · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $5–$1,300
- DIY difficulty
- Moderate
P0508 applies to engines with a separate idle air control (IAC) valve, usually a solenoid or rotary type bolted to the throttle body. In the most common layout the valve gets battery power from a fused feed and the engine computer switches its ground side on and off many times a second. When the driver is off, the computer expects to see roughly battery voltage on that control wire; P0508 sets when it sees low voltage instead. That happens when the valve coil is shorted, when the power feed to the valve is lost, or when the control wire is rubbing against ground. Drive-by-wire engines without a separate IAC valve normally don't use this code.
Can I keep driving? Usually drivable, but idle can be stuck high or the engine can stall at stops depending on how the circuit failed. Keep your foot ready on the brake and be prepared for heavy steering if it dies.
How to read P0508
- P Powertrain
- 0 Generic (SAE)
- 5 Speed, idle & inputs
- 08 Specific fault
Symptoms
- Idle stuck high, sometimes 1,500 rpm or more, on some failures
- Or engine stalls at stops and needs throttle to keep running
- Hard starting when cold, fine once moving
- Idle doesn't change when the A/C or lights are switched on
- Check engine light on, often with P0505 or P0506
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- IAC valve coil shorted internally from heat and age
- Lost power feed to the valve – blown fuse, failed main relay circuit or open supply wire
- Control wire chafed or pinched to ground near the intake or engine brackets
- Water or coolant in the IAC connector
- Damaged driver in the engine computer, often after a shorted valve
- Wrong replacement valve with incorrect coil resistance
Diagnosis step by step
Confirm the IAC layout
Check the wiring diagram: count the wires at the valve and see which one carries battery power and which one goes to the computer. P0508 testing depends on that layout, and some engines use a three-wire rotary valve with two control lines.
Tools: Flashlight
Check the power feed first
Key on, valve connected, back-probe the supply pin – it should show battery voltage. No voltage points to a fuse, the main relay circuit or an open wire, and the computer will read the control wire as low even though the valve is fine.
Tools: Multimeter
Measure coil resistance
Unplug the valve and measure across its terminals. Many solenoid and rotary IAC valves fall somewhere around 8–25 ohms per winding depending on design; a reading of just a few ohms or less means a shorted coil.
Tools: Multimeter
Test the control wire for a short to ground
Battery disconnected, valve and computer both unplugged, measure from the control wire to engine ground. Any continuity means damaged insulation; check where the harness passes brackets, the intake manifold and hot coolant hoses.
Tools: Multimeter
Protect the computer when replacing parts
A shorted IAC coil can overload the computer's driver. Measure the new valve's resistance before plugging it in, and if a driver has failed, replace the valve along with the computer so the new one isn't damaged too.
Tools: Multimeter
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Replace blown fuse and find the cause | Yes | $5–$120 |
| Repair control or supply wiring | No | $90–$300 |
| Replace idle air control valve | Yes | $150–$400 |
| Dry and reseal IAC connector, fix coolant leak | Yes | $50–$250 |
| Repair or replace engine computer | No | $500–$1,300 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Emissions / smog test impact
Will fail
The check engine light is on, so an OBD smog check fails. On older vehicles still tested at the tailpipe, a stuck-high idle can also affect idle emissions readings.
Frequently asked questions
Why is my idle stuck high with a 'low' circuit code?
"Low" describes the voltage on the control wire. If that wire is shorted to ground, the valve can be held fully energized – wide open on many designs – so the engine idles fast even though the code says low.
My engine has an electronic throttle – can it set P0508?
Usually not. Drive-by-wire engines control idle with the throttle motor and use throttle actuator codes instead. If your drive-by-wire engine shows P0508, check the manufacturer's definition, as it may be used differently.
Can a bad IAC valve damage the engine computer?
Yes. A coil with very low resistance draws more current than the driver is designed for. That's why you should measure the valve before replacing the computer and never reuse a shorted valve.
Is P0508 the same as P0511?
Both concern the IAC circuit, but P0508 specifically reports low voltage – a short or lost feed. P0511 is the general circuit fault, where opens and connector problems are more typical.