Number P0505: Idle Control System
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 itse
Can you drive with P0505?
Severity: Plan the repair. Yes — this is an emissions-system fault. The car will generally drive normally, but it will fail an emissions inspection with the light on.
If you ignore it: Fuel economy and emissions creep up, and some faults in this family (rich/lean running) stress the catalyst over time.
This is the #42 most common code in our top-150 library, in the idle & throttle control family (Powertrain system). Severity is Number Nest's editorial triage — if the light is flashing or the car feels unsafe, stop driving it whatever any page says.
What P0505 means
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.
Systems involved: idle control, throttle body, idle air control valve, engine control module (ECM/PCM).
Causes, ranked by likelihood
| # | Cause | Likelihood |
|---|---|---|
| 1 | Throttle body or idle valve dirty or gummed | most likely |
| 2 | False air (hoses, intake gasket, crankcase ventilation) | possible |
| 3 | Idle valve faulty mechanically or electrically | possible |
| 4 | Learned values not reset after cleaning | less common |
Check them in order — the list is ranked so the likely, cheaper causes come before the expensive part.
Symptoms you may notice
- Idle speed fluctuates, too high or too low
- Engine stalls at idle or when coasting
- Start-stop not working
Misdiagnosis traps
- Cleaning the throttle bore fixes most idle faults on cable-throttle cars; replacing the IAC motor without cleaning buys the same dirt twice.
- A vacuum leak raises idle beyond the IAC's authority — the code blames the idle system for an air leak elsewhere.
How a good shop confirms it
- Clean the throttle bore and IAC passage, verify no vacuum leaks, then test the motor's movement.
Repair options & costs
| Repair | Shop total (parts + labor) | DIY (parts only) | Difficulty |
|---|---|---|---|
| Idle air control valve / throttle service | $130 – $365 | $80 – $240 (parts only) | 2/5 |
| Throttle body cleaning + relearn | $60 – $173 | $10 – $60 (parts only) | 1/5 |
National ranges from Number Nest's cost model (midsize car, independent shop, US-average labor).
OBDex repair data for this code rates the job difficulty medium with a typical labor time in the 0.5–2 hour range and DIY possible: yes.
Price this repair in your state
Estimated total: — (midsize car, independent shop)
Frequently asked questions
Why does the idle surge up and down?
The computer is hunting: it closes the idle valve, RPM falls too far, it overcorrects. Surging almost always means extra air entering somewhere it should not.
How much does it cost to fix P0505?
The fix most associated with this code — idle air control valve / throttle service — runs $130 – $365 at an independent shop for a midsize car in our cost model (parts + labor, US average). The code names a system, not a part, so confirm the cause before spending: the ranked causes above are ordered by likelihood.
Can I drive with P0505?
See the can-you-drive verdict above — it is based on the system involved and the cascading-damage risk, and it is our editorial triage, not a manufacturer instruction. If the light is flashing or the car feels unsafe, stop driving it regardless of any code.
Will P0505 clear itself?
If the fault was a one-off (a loose cap, a cold-start glitch), the light can go out after several clean trips and the code ages into history. A fault that is still present re-sets the code as soon as the car runs that self-test again — monitors run under specific conditions, which is why "it hasn't come back in two days" is not proof of a fix.
Will my car fail inspection with P0505?
In states with OBD-based emissions inspection, yes — an illuminated check-engine light or an incomplete readiness monitor fails the test regardless of what the tailpipe would show. C-codes (ABS) fall under safety inspection instead in states that have one.
What should I check first for P0505?
The first item on the ranked-causes list above — it is ranked likelihood-first for exactly this reason — plus the free checks: recent work in the area (connectors left loose are a classic), fluid levels, and the battery and grounds on electrical codes.
Where this data comes from
Code facts, causes and symptoms come from OBDex — an open dataset of independently authored descriptions of the standardized generic OBD-II codes (CC0 1.0, public domain; github.com/foerbsnavi/OBDex). The SAE J2012 document itself is copyrighted and is not reproduced here. Cost ranges are Number Nest's own US planning model: national parts and labor ranges cross-checked against the CarMD Vehicle Health Index 2026 published averages, with labor adjustable by the BLS OEWS May 2025 state wage index for auto technicians (occupation 49-3023) — a wage index, not a surveyed shop rate. Severity and can-you-drive notes are our editorial triage, not a manufacturer instruction.