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

P0508 Idle Air Control System Circuit Low

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. P0508 is an electrical fault in the drive circuit of a separate idle valve. Subaru (also in the Saab 9-2X) feeds the valve from the main relay and pulses it to ground; the module counts signal edges and sets P0508 at 8 or fewer in 64 ms instead of about 160 – the output hangs at one level – with the warning lamp at once and limp mode without idle control. Many makers, Ford among them, report all circuit faults under P0511 instead.

Likely causes

Idle valve coil faulty
Control wire shorted to ground or open
Supply missing (main relay, fuse)
Connector contact at valve or module
Driver in the engine module

Symptoms

  • Idle fluctuates or is wrong
  • Engine stalls 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

P0509P0511P0505P0506P0507

References: saabwisonline.com, saabwisonline.com, hella.com.