P0509 Code: Idle Air Control System Circuit High
The ECU sees too much voltage on the idle air valve circuit, so it has lost control of the idle bypass.
At a glance
Severity: Low
- Meaning
- Idle Air Control System Circuit High
- System
- Speed & Idle Control · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $40–$1,300
- DIY difficulty
- Moderate
P0509 belongs to engines that still meter idle air through a separate valve beside the throttle plate rather than through an electronic throttle. The ECU checks the voltage on the valve's control pin against what its own output should produce, and P0509 sets when that voltage reads high. What 'high' means depends on which side the ECU switches. If it grounds the valve, a control wire touching a 12-volt feed keeps the pin high even when the driver is on. If it supplies the valve, an open coil or a broken valve ground leaves the pin unloaded and it floats high. Either way the valve drops to its spring-set rest position, and on two-coil rotary valves it can swing hard to one end, giving a racing idle or an engine that dies as you coast to a stop.
Can I keep driving? Drivable, but expect stalling when you stop or switch on the A/C, and keep a foot ready on the gas at intersections. If the idle races, it will make automatic transmissions creep hard, so hold the brake firmly.
How to read P0509
- P Powertrain
- 0 Generic (SAE)
- 5 Speed, idle & inputs
- 09 Specific fault
Symptoms
- Idle fixed at one speed, cold or warm, and ignoring the A/C and headlights
- Idle races well above normal on engines with two-coil rotary valves
- Stalls when rolling to a stop or when the A/C clutch engages
- Needs a touch of throttle to start and stay running when cold
- Check engine light, sometimes with an idle rpm code
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Control wire chafed against a 12-volt feed in the harness near the intake or fuse box
- Open winding in the idle air control valve
- Broken or corroded ground wire or ground point for the valve
- Faulty splice from an aftermarket alarm or remote starter
- Corroded or loose terminals in the valve connector
- ECU output stage stuck off
Diagnosis step by step
Find out which side the ECU switches
Look at the wiring diagram and note whether the ECU drives the ground or the supply of the valve, and whether the valve has two pins or three. The rest of the test depends on it: with ground switching, look for unwanted voltage; with supply switching, look for an open circuit.
Tools: OBD-II scanner
Watch the idle respond to a command
Warm engine, idle: command the idle valve or desired idle up and down with a scan tool. If rpm doesn't change at all and the commanded value moves, the valve isn't being driven; if rpm swings to an extreme, one coil of a rotary valve is dead.
Tools: OBD-II scanner
Check the control pin with the valve unplugged
Key on, valve disconnected, measure the control pin at the harness to ground. On a ground-switched design it should read near 0 volts with the valve unplugged; battery voltage here means the wire is shorted to a power feed.
Tools: Multimeter
Measure the valve windings
Measure each winding and compare with the published value for your valve. An open reading on any winding condemns the valve; on three-pin rotary valves, the two outer pins to the center pin should read nearly the same.
Tools: Multimeter
Test the ground and look for add-on wiring
Measure voltage drop from the valve's ground pin to the battery negative while it's commanded on; more than a few tenths of a volt means a bad ground. Also look for alarm or remote-start splices in the harness near the steering column or ECU.
Tools: Multimeter, Flashlight
Relearn idle after the fix
After repairing the circuit, clear the code and let the engine idle warm with all loads off for several minutes so the ECU can relearn its idle position. Some engines need a specific relearn procedure from the service information.
Tools: OBD-II scanner
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 |
|---|---|---|
| Repair shorted or broken valve wiring | No | $90–$280 |
| Replace idle air control valve | Yes | $120–$400 |
| Clean and repair ground point | Yes | $40–$120 |
| Remove or redo faulty alarm/remote-start splices | No | $80–$250 |
| ECU repair or replacement | 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
Any stored P0509 means an active check engine light, which fails OBD-II emissions testing in states that do it. Older vehicles that are tested with a tailpipe probe instead can also fail if the idle speed is out of range.
Frequently asked questions
Why is my idle stuck high with P0509?
On two-coil rotary idle valves, one coil opens the valve and the other closes it. If the closing side loses its circuit, the opening side wins and the idle races; the reverse causes stalling.
What's the difference between P0508 and P0509?
Both concern the same valve circuit. P0508 means the ECU sees too little voltage, typically a lost feed or a wire to ground; P0509 means too much, usually a wire touching a power feed or an open coil on a supply-switched design.
Will cleaning the idle valve fix P0509?
Not on its own. P0509 is an electrical fault, so a carbon-clogged valve sets idle rpm codes instead. Cleaning is still worthwhile if you replace wiring, since a sticky valve can make the relearn misbehave.
My car has an electronic throttle. Can it set P0509?
Usually not. Drive-by-wire engines control idle with the throttle motor and report their problems with throttle actuator codes. P0509 is mostly seen on older engines with a separate idle valve.