P0505 Code: Idle Air Control System Malfunction
The engine computer can't hold idle speed where it wants it – usually a dirty or sticking idle control.
At a glance
Severity: Low
- Meaning
- Idle Air Control System Malfunction
- System
- Speed & Idle Control · Powertrain
- Safe to drive?
- With caution
- Typical repair cost
- $10–$900
- DIY difficulty
- Moderate
Your engine control module (ECM) sets a target idle speed based on coolant temperature, electrical load, A/C demand and whether the transmission is in gear. It reaches that target by opening or closing an idle air control (IAC) valve or, on drive-by-wire engines, by cracking the electronic throttle plate. P0505 sets when the ECM commands a correction but actual rpm doesn't follow, or when it sees a fault in the idle control circuit. Carbon buildup in the throttle body or IAC passage, a vacuum leak and a sticking IAC valve cause most cases; wiring faults and a bad ECM are far less common.
Can I keep driving? Usually drivable, but the engine may stall when you roll up to a stop or switch on the A/C, and steering gets heavy if it does. Leave extra room in traffic and get it fixed soon.
How to read P0505
- P Powertrain
- 0 Generic (SAE)
- 5 Speed, idle & inputs
- 05 Specific fault
Symptoms
- Idle hunts up and down or surges at stoplights
- Engine stalls when coming to a stop, especially with the A/C on
- Low, shaky idle right after a cold start
- Hard starting unless you press the gas pedal slightly
- Idle stays too high after the engine warms up
- Check engine light on
Possible causes
Ranked by how often each cause is behind this code (editorial estimate).
- Carbon and varnish buildup in the throttle body bore or IAC air passage
- Sticking or worn IAC valve (stepper motor or rotary solenoid)
- Vacuum leak – cracked hose, PCV hose or leaking intake gasket
- Corroded connector or broken wire in the IAC circuit
- Faulty electronic throttle body on drive-by-wire engines
- Idle not relearned after a battery disconnect or throttle cleaning
- ECM driver circuit fault
Diagnosis step by step
Read freeze frame and any other codes
Note coolant temperature, rpm and load when P0505 set. Lean codes such as P0171 alongside it point toward a vacuum leak, while throttle position codes point at the throttle body.
Tools: OBD-II scanner
Watch idle data live
With the engine fully warm, compare desired and actual idle rpm along with IAC counts or throttle angle. If the ECM is driving the IAC to its limit and rpm still won't follow, the air path is restricted or leaking rather than the command being wrong.
Tools: OBD-II scanner
Hunt for vacuum leaks
Listen for hissing and check the PCV hose, brake booster hose and intake boot for cracks. Leaks at intake gaskets are often silent; a smoke tester finds them in minutes.
Tools: Smoke machine, Flashlight, Vacuum gauge
Clean the throttle body and IAC passage
Engine off, remove the intake boot and wipe carbon out of the throttle bore and idle bypass port with throttle body cleaner and a rag. On electronic throttles, don't force the plate open and don't soak the motor housing with cleaner.
Tools: Screwdriver, Soft brush, Gloves
Test the IAC valve and its wiring
On engines with a separate IAC, unplugging it at warm idle should change rpm noticeably; no change suggests a stuck valve. Check winding resistance and the harness against the service spec and look for green corrosion in the connector.
Tools: Multimeter
Run an idle relearn and verify
After cleaning or a battery disconnect, let the ECM relearn idle – on many engines by idling fully warm with all accessories off for several minutes, on others with a scan-tool routine. Clear the code and road test, including stops with the A/C on.
Tools: OBD-II scanner
Where the parts are
- 1Ignition coil
- 2Spark plug
- 3Fuel injector
- 4Camshaft sensor
- 5Crankshaft sensor
- 6Engine computer (ECM)
- 7Intake manifold
Repair options & costs
| Repair | DIY? | Estimated cost |
|---|---|---|
| Clean throttle body and IAC valve yourself | Yes | $10–$25 |
| Throttle body cleaning and idle relearn at a shop | No | $90–$180 |
| Replace IAC valve and gasket | No | $150–$400 |
| Repair vacuum leak (hose, PCV, intake gasket) | No | $50–$450 |
| Replace electronic throttle body | No | $350–$900 |
Rough US estimates for parts and labor; shop rates vary by region. Not a quote.
Notes by brand
Honda
Many 1990s–2000s Hondas and Acuras use a separate idle air control valve that gums up with carbon. Cleaning or replacing it, followed by Honda's idle relearn (warm idle with all loads off until the radiator fan cycles), clears most P0505 cases.
Ford
Fords with a bolt-on IAC valve on the throttle body frequently set P0505 from a carboned-up valve or a leaking valve gasket. Cleaning or replacing the valve and gasket is the usual fix.
Field data
| Toyota | 2 | Camry, RAV4 |
|---|
Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.
Emissions / smog test impact
Will fail
The check engine light is on, so an OBD-based smog or emissions test will fail. After the repair, drive normally for a few days so the readiness monitors complete before you retest.
Frequently asked questions
Can I clean the IAC valve instead of replacing it?
Often yes. If the valve still moves freely and its resistance is within spec, carbon is the usual culprit and cleaning restores idle control. Replace it if it is electrically open or shorted, or still sticks after cleaning.
Why does my car only stall with the A/C on?
The A/C compressor adds load at idle, so the ECM has to let in more air to keep rpm up. A dirty or weak idle control can't supply that extra air fast enough, so rpm dips and the engine dies.
My engine has no IAC valve – why P0505?
On drive-by-wire engines the throttle motor controls idle directly. There the code usually means carbon in the throttle bore, a vacuum leak or a problem with the electronic throttle body.
Is P0505 the same as P0507?
No. P0505 is a general idle control fault – the ECM can't control idle or sees a circuit problem. P0507 specifically says idle is higher than the target, which usually points to extra air leaking in.