P0505: Idle Air Control System
The engine computer cannot control the idle speed properly, so the idle is too high, too low or unstable.
What P0505 means
Your engine needs a little extra air at idle to stay running, and more when loads like the A/C or alternator kick in. RepairPal explains that older cars use an idle air control motor or stepper valve that bypasses air around the closed throttle plate, while newer drive-by-wire cars crack the throttle open with its own motor. P0505 means the computer commanded an idle change and the RPM did not respond the way it expected. Carbon buildup in the throttle body or idle valve is a very common cause, along with a failed valve, vacuum leaks and wiring problems. Symptoms include a low idle that dies at stops, a high or hunting idle, and stalling when the A/C switches on. KBB notes these codes do not usually prevent driving, but stalling in traffic is a real hazard. Shops clean the throttle body and idle passages, smoke test for vacuum leaks, command the idle valve with a scan tool and run any required idle relearn.
Common symptoms
- Check engine light on
- Very low idle that may stall at stops
- High or surging idle
- Rough idle
- Stalling when the A/C turns on
Most common causes
- Dirty or failed idle air control valve or motor
- Carbon buildup in the throttle body
- Vacuum leak
- Faulty electronic throttle body on drive-by-wire cars
- Wiring problem to the idle control
- Battery recently disconnected and idle not relearned
What to check yourself first
Clean the throttle body with throttle body cleaner if your owner manual allows it, since carbon is a very common cause. If the battery was recently disconnected, look up the idle relearn procedure for your car. Listen for vacuum leaks with the engine idling.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace throttle body | $623-$766 | On drive-by-wire cars, idle is controlled by the electronic throttle body. |
| Smoke test for vacuum leaks | $61-$90 | Finds vacuum leaks that the idle control cannot compensate for. |
| Check engine light diagnosis at a shop | $125-$183 | Includes throttle body cleaning and an idle relearn, which fix many idle codes. |
| Replace PCV valve | $124-$173 | If a stuck PCV valve is acting as a vacuum leak. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0506 Idle Air Control System RPM Lower Than Expected
- P0507 Idle Air Control System RPM Higher Than Expected
- P0171 System Too Lean (Bank 1)
- P0121 Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance
- P2135 Throttle/Pedal Position Sensor/Switch "A"/"B" Voltage Correlation