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

P0505 Idle Control System

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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. P0505 is the general fault of the idle control, used where the maker does not separate too low from too high or the control is at its limit. With an idle valve it is typically sticking or gummed (Saab T5: idle outside 700–1200 rpm); on electronic throttles a Ford investigation shows P0505 from deposits in the throttle body. Ford's 2020–23 engines disable start-stop with P0505–P0507. Warning lamp behaviour depends on the maker.

Likely causes

Throttle body or idle valve dirty or gummed
False air (hoses, intake gasket, crankcase ventilation)
Idle valve faulty mechanically or electrically
Learned values not reset after cleaning

Symptoms

  • Idle speed fluctuates, too high or too low
  • Engine stalls at idle or when coasting
  • Start-stop not working

Repair estimate

DifficultyMediumDIY possible
Parts and labour€40–600
Time0.5–2 h

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

What to check first

  1. Clean the throttle body
  2. Check for vacuum leaks
  3. Perform an idle relearn if required

Parts involved

idle controlthrottle bodyidle air control valveengine control module

Related codes

P0506P0507P0511P050AP2111P2112

References: static.nhtsa.gov, saabwisonline.com, hella.com, roushcleantech.com.