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

P0518 Idle Air Control Circuit Intermittent

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

Older engines without electronic throttle have a separate idle air control valve bypassing the closed throttle; CARB requires its output to be checked but not the fault types to be coded separately. P0518 means its circuit fails sporadically – the signal or current briefly leaves its valid range and returns. No manufacturer document available to us uses P0518; GM and Ford report idle valve faults as P0505–P0507 and P0511. Engines with electronic throttle do not set it.

Likely causes

Loose contact or corrosion at the idle valve connector
Broken or chafed wire in the harness
Valve coil with shorted turns (temperature-dependent)
Driver in the engine module

Symptoms

  • Fluctuating idle
  • Engine stalls when coasting or stopping
  • Check engine light (depending on the maker)

Repair estimate

DifficultyEasyDIY possible
Parts and labour€0–200
Time0.5–2 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

P0505P0506P0507P0508P0509P0511

References: law.cornell.edu, hella.com, newtonnet.co.uk.