Powertrain, generic · Vehicle speed, idle control and auxiliary inputs

P0509 Idle Air Control System Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

CARB requires the idle speed control to be monitored: petrol engines must flag a fault if the target idle speed is not reached within +200/−100 rpm, diesels within ±30 %. Older engines use a separate idle air control valve that bypasses the closed throttle; with electronic throttle the throttle itself controls idle and no separate valve exists. P0509 is the counterpart of P0508: the drive circuit of the idle valve is shorted to positive or shows implausibly many signal edges – Subaru sets it at 225 or more in 64 ms, with the warning lamp at once and limp mode without idle feedback. It only exists on engines with a separate idle valve.

Likely causes

Control wire shorted to positive
Idle valve with shorted turns
Connector corroded or wet, interference
Engine module faulty

Symptoms

  • Idle too high or fluctuating
  • Stalling at idle
  • Check engine light on at once

Repair estimate

DifficultyMediumDIY possible
Parts and labour€50–350
Time0.5–1.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

idle air control valvevalve wiringengine control module

Related codes

P0508P0511P0505P0506P0507

References: saabwisonline.com, hella.com, law.cornell.edu.