# P0505: Idle Air Control System

**Verdict:** Drive to a shop this week

Speed and idle control - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996.

- **Typical cost:** $80-$900
- **Urgency:** This week
- **Most likely:** Dirty or failed idle air control valve
- **DIY?:** Steps 1-2

## Quick answer

P0505 means the engine computer cannot control idle speed the way it wants to. On older cars this usually points to a dirty or failed idle air control valve, while on newer cars with electronic throttles it often points to a dirty throttle body. Vacuum leaks can also cause it.

## What does P0505 mean?

At idle your foot is off the gas, so the engine computer controls idle speed itself by letting a small, measured amount of air past the closed throttle. Older engines use a separate idle air control (IAC) valve, a small motor-driven valve on or near the throttle body. Newer engines with electronic throttles move the throttle plate itself to adjust idle air. The computer raises idle for a cold engine, the AC compressor, and other loads.

P0505 sets when actual idle speed does not follow the target the computer is commanding or, on some makes, when the IAC valve circuit itself does not respond correctly. The test usually runs with the engine warm, the car stopped, and no other faults present. How far idle may stray and for how long varies by maker, and many cars need the fault on two drive cycles before the check engine light comes on.

## What are the symptoms of P0505?

- Idle speed too high, too low, or surging up and down
- Engine stalls when you stop or turn on the AC
- Hard starting
- Check engine light on

## What causes P0505?

Ranked from most to least likely. Check them in this order: the cheap, common causes first.

1. **Dirty or carbon-clogged idle air control valve or throttle body** (most common). Oil vapor from the PCV system and a little exhaust soot bake onto the throttle plate and into the narrow idle air passage. As the opening shrinks, the computer runs out of range to hold idle at its target. Carbon builds slowly with mileage on most gasoline engines, and frequent short trips speed it up.
2. **Failed idle air control valve** (common). Separate IAC valves use a small stepper motor or rotary solenoid that moves constantly while the engine idles. Over the years the windings can fail or the plunger can stick in carbon, so the valve no longer moves when commanded. It mostly affects older engines from the 1990s and 2000s, and idle often stays stuck high or low.
3. **Vacuum leak** (common). A cracked vacuum hose, a loose intake duct after the airflow sensor, or a leaking intake manifold gasket lets in air the computer did not plan for. Idle runs high, and the computer cannot bring it down to target. Rubber parts harden with age, so older engines are most affected, and a hiss at idle is a clue.
4. **Wiring or connector fault at the valve** (less common). The IAC valve or electronic throttle connects through a plug on top of the engine that lives through constant heat cycles and oil mist. A loose, corroded, or oil-soaked connector, or a broken wire inside the harness, stops commands from reaching the valve. Gently wiggling the connector at idle and watching for RPM changes can reveal it.
5. **Electronic throttle body failure** (less common). On drive-by-wire engines the throttle body has its own small motor, plastic gears, and position sensors. Worn gears, a weak motor, or a failing sensor can stop it from setting the very small openings used at idle. This usually brings throttle position or throttle actuator codes too, and the car may drop into a reduced-power mode.

## How do you diagnose P0505?

1. Check for other codes, such as vacuum leak or throttle position codes.
2. Look at the throttle body and idle air passage for carbon buildup.
3. Listen and check for vacuum leaks with the engine running, or have a smoke test done on the intake.
4. Command the idle air control valve with a scan tool and watch whether idle speed changes.

## How do you fix P0505, and what does it cost?

| Fix | Typical cost | What it involves |
| --- | --- | --- |
| Clean throttle body and idle air passage | $80-$180 | Some cars need an idle relearn with a scan tool after cleaning. |
| Replace idle air control valve | $150-$400 | For older cars with a separate idle air control valve. |
| Repair vacuum leak | $100-$400 | Hoses are cheap. Intake gaskets cost more in labor. |
| Replace electronic throttle body | $350-$900 | For newer cars where the throttle body itself controls idle. |

## Is it safe to drive with P0505?

> **Drive to a shop this week.** The car is usually drivable, but stalling at stops or in turns can be dangerous. Get it fixed soon if the engine stalls.

## Will P0505 fail an emissions test?

In most OBD-II inspection states, a lit check engine light with P0505 stored is a fail. Idle problems do not block the readiness monitors directly, but clearing the code resets all of them, so the car may be turned away as not ready. A vacuum leak behind P0505 can also set lean codes that fail the test.

## After the repair: clearing P0505

After cleaning the throttle body or replacing the valve, clear the code and run whatever idle relearn your vehicle requires. Some cars relearn by idling for several minutes with all accessories off, while others need a scan tool. A fixed car holds a steady idle both cold and warm, with the AC on and off, without stalling or surging.

## How to prevent P0505

- Replace the engine air filter on schedule so dirt does not reach the throttle and idle passages.
- Clean the throttle body when idle first starts to roughen, using cleaner labeled safe for throttle bodies.
- Keep the PCV valve and its hoses in good shape, since oily crankcase vapor builds carbon at the throttle.
- Replace cracked or soft vacuum hoses as soon as you notice them, before they leak and upset idle.
- After disconnecting the battery, let the engine idle to relearn before driving if your manual says to.

## P0505 by make

| Make | What to know |
| --- | --- |
| Honda | Many older Honda and Acura models with a separate idle air control valve set P0505 when the valve gets dirty or fails, and cleaning or replacing it is a common fix. |

## Codes often seen with P0505

- [P0506](/codes/p0506): Idle Air Control System RPM Lower Than Expected
- [P0507](/codes/p0507): Idle Air Control System RPM Higher Than Expected
- [P0171](/codes/p0171): System Too Lean (Bank 1)
- [P0121](/codes/p0121): Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance

## Frequently asked questions

### Can I drive with a P0505 code?

Usually yes, if the engine is not stalling. If it stalls at stops, in turns, or when the AC kicks on, repair it soon, because a stalled engine loses power steering assist and brake assist fades after a few presses. A steady high idle is less dangerous but still worth fixing.

### Will cleaning the throttle body fix P0505?

It often does, because carbon buildup is the most common cause of P0505. A shop cleaning of the throttle body and idle passage costs about $80 to $180, or a can of cleaner if you do it yourself. If idle stays wrong afterward, test the IAC valve and look for vacuum leaks.

### Do I need an idle relearn after fixing P0505?

Many cars need an idle relearn after the throttle body is cleaned or the IAC valve is replaced, because the computer has learned settings for the old, dirty airflow. Some relearn on their own after idling for several minutes or a few drive cycles, while others need a scan tool, as your service manual explains.

### How much does it cost to fix P0505?

Most P0505 repairs cost $80 to $900. Cleaning the throttle body runs about $80 to $180, a new IAC valve $150 to $400, and a vacuum leak repair $100 to $400. A new electronic throttle body, needed only when cleaning fails and it tests bad, costs about $350 to $900.

### What is the difference between P0505, P0506, and P0507?

P0505 is the general idle control code, meaning idle is not following the computer's target or the valve circuit has a fault. P0506 means idle is lower than expected, and P0507 means it is higher. Low idle points toward carbon or extra load, while high idle points toward vacuum leaks.

### Can a vacuum leak cause P0505?

Yes, a vacuum leak is a common cause. Unmetered air pushes idle speed above what the computer can pull back down, so it cannot reach its target. Leaks often set lean codes such as P0171 as well. A smoke test of the intake is the most reliable way for a shop to find one.

### Can I fix P0505 myself?

Often, yes. Cleaning the throttle body and idle air passage is a common DIY job on many cars, and checking vacuum hoses needs only basic tools. Replacing a bolt-on IAC valve is also manageable for most owners. Electronic throttle bodies and some idle relearn procedures may need a scan tool.

### Why does my car stall when I turn on the AC?

The AC compressor adds load, so the computer must open the idle air control to let in more air and keep idle steady. If the IAC valve or throttle body is dirty or slow, that extra air does not arrive in time and the engine stalls. It is a classic P0505 symptom.

### Will P0505 fail an emissions test?

Yes, in most states a lit check engine light with P0505 stored fails an OBD-II emissions test. Clearing the code resets the readiness monitors, so the car may be rejected as not ready if you test too soon. Fix the idle problem, then drive normally for several days before testing.

### Can a weak battery or alternator affect idle?

It can make idle unstable on some cars, though it rarely sets P0505 on its own. Low voltage can make an IAC valve or electronic throttle respond poorly, and a struggling alternator adds load at idle. If idle trouble comes with dim lights or slow cranking, have the charging system tested too.

## Sources

- [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/)

## Other speed and idle control codes

- [P0500 Vehicle Speed Sensor "A"](https://diagxcar.com/codes/p0500)
- [P0506 Idle Air Control System RPM Lower Than Expected](https://diagxcar.com/codes/p0506)
- [P0507 Idle Air Control System RPM Higher Than Expected](https://diagxcar.com/codes/p0507)
- [P0520 Engine Oil Pressure Sensor/Switch "A" Circuit](https://diagxcar.com/codes/p0520)
- [P0521 Engine Oil Pressure Sensor/Switch "A" Circuit Range/Performance](https://diagxcar.com/codes/p0521)
- [P0522 Engine Oil Pressure Sensor/Switch "A" Circuit Low](https://diagxcar.com/codes/p0522)
- [P0523 Engine Oil Pressure Sensor/Switch "A" Circuit High](https://diagxcar.com/codes/p0523)
- [P0524 Engine Oil Pressure Too Low](https://diagxcar.com/codes/p0524)
- [P0562 System Voltage Low](https://diagxcar.com/codes/p0562)
- [P0571 Brake Switch "A" Circuit](https://diagxcar.com/codes/p0571)
- [P0121 Throttle/Pedal Position Sensor/Switch "A" Circuit Range/Performance](https://diagxcar.com/codes/p0121)
- [P0122 Throttle/Pedal Position Sensor/Switch "A" Circuit Low](https://diagxcar.com/codes/p0122)
- [P0123 Throttle/Pedal Position Sensor/Switch "A" Circuit High](https://diagxcar.com/codes/p0123)
- [P0402 Exhaust Gas Recirculation "A" Flow Excessive Detected](https://diagxcar.com/codes/p0402)

## Symptoms

- [Why is my car idling rough?](https://diagxcar.com/symptoms/rough-idle)
- [Why does my car stall while driving?](https://diagxcar.com/symptoms/car-stalls-while-driving)
- [Why does my car shake at idle?](https://diagxcar.com/symptoms/car-shakes-at-idle)

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Source page: https://diagxcar.com/codes/p0505
Updated: 2026-11-03
General information, not a substitute for a qualified mechanic.