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

P0506 Idle Control System RPM - Lower Than Expected

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. P0506 means the idle speed of the warm engine at standstill stays too low despite fully used control. GM uses more than 91 rpm filtered for 10 s (MIL after two trips), Ford petrol −100 rpm plus a correction above 1 lb/min, Ford diesel −100 rpm for 5 s, Subaru −100 rpm with the correction at its limit for 10 s three times. Typical are deposits, extra load or a restricted intake or exhaust.

Likely causes

Deposits at the throttle or bypass channel, or gummed idle valve
Extra load: stiff accessory drive, gearbox or converter
Intake or exhaust restricted (air filter, catalyst)
Idle valve or throttle actuator faulty, wrong learned value
Mechanical engine damage (low compression)

Symptoms

  • Idle too low, engine shakes
  • Engine stalls when stopping or with A/C on
  • Check engine light after two trips

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–600
Time0.5–2 h

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

What to check first

  1. Clean throttle body
  2. Check air filter
  3. Perform idle relearn

Parts involved

idle controlthrottle bodyidle air control valveengine control module

Related codes

P0505P0507P050AP2111P0171

References: gsi.ext.gm.com, saabwisonline.com, saabwisonline.com, newtonnet.co.uk.