P0505 – Idle Control System Malfunction (OBD2)
Fault code P0505 means "Idle Control System Malfunction". Below are the real-world causes, step-by-step tests and OEM part numbers.
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P0505 quick summary
| Fault code | P0505 |
|---|---|
| Meaning | Idle Control System Malfunction |
| System | Powertrain (Engine & Transmission) |
| Vehicles | Generic OBD2 |
| Severity | Low |
| Can I drive? | Yes, but the idle may be too high, too low or unstable and the engine can stall at junctions. |
| Repair cost | €10 – €450 |
Symptoms of P0505
- Idle speed too high, too low or hunting up and down
- Engine stalls when coming to a stop or when the A/C switches on
- Rough idle
- Check Engine light on
Most common causes of P0505 (most likely first)
Mechanic's Pro Tip
After a battery change or throttle body cleaning, many cars need an idle / throttle adaptation (basic setting) – without it, P0505 can appear even though nothing is broken. Otherwise a carbon-coated throttle body or a vacuum leak at the crankcase ventilation is the usual cause.
1.Carbon build-up on the throttle body or idle air passages
Most likely2.Dirty or faulty idle air control valve (older engines)
3.Vacuum leak (intake hoses, crankcase ventilation / PCV valve)
4.Throttle adaptation (basic setting) not done after battery or throttle work
5.Faulty electronic throttle body or damaged wiring
Step-by-step diagnostic guide
- 1
Read all codes – fix vacuum, MAF or throttle codes first.
- 2
Compare specified and actual idle speed in live data.
- 3
If the fault appeared after battery work, perform the idle / throttle adaptation for your vehicle.
- 4
Check the intake and crankcase ventilation system for leaks (smoke test).
- 5
Inspect the throttle body for carbon and clean it with throttle body cleaner, then perform the adaptation.
- 6
Check the throttle body / idle valve connector and wiring.
Frequently asked questions
Will cleaning the throttle body fix P0505?
Often yes, if carbon is the cause. Always perform the throttle adaptation afterwards, otherwise the idle may still be wrong.
How much does it cost to fix P0505?
Throttle body cleaner costs about 10 €. A new throttle body typically costs 100–300 € for the part and 150–450 € fitted.
Related fault codes
This guide is for information only. Working on vehicles can be dangerous — if you are unsure, have the car checked by a qualified mechanic. Values can differ between model years; always check the manufacturer's repair information.