P0505 means the engine control module has detected a general malfunction in the idle control system without the system being able to classify it as specifically too high or too low — distinct from P0507 (idle RPM higher than expected) and P0506 (idle RPM lower than expected), which both identify a specific direction of fault. In practice this often shares the same root causes as those two more specific codes — the idle air control valve, throttle body, or related wiring — but the diagnostic starting point is broader since the direction of the fault hasn’t been pinned down.
What Does P0505 Mean?
The idle control system continuously monitors and adjusts idle speed to hit its target, and P0505 fires when the system detects something wrong with this process without being able to definitively classify it as a too-high or too-low condition. See our P0507 and P0506 guides for the two direction-specific variants of essentially this same underlying fault — P0505 is the general, unclassified version, similar in concept to how P0440 serves as the general EVAP code alongside its more specific size-classified siblings on this site.
Common Symptoms
- An unstable, rough, or inconsistent idle
- The engine occasionally stalling at idle — see our car dies at idle guide
- Check engine light on
- Idle speed that fluctuates rather than settling at a consistent RPM
Common Causes, Most Likely First
- A dirty or carbon-fouled idle air control valve or throttle body, the same leading cause covered in our P0507 and P0506 guides, since debris can cause the system to behave inconsistently rather than settling clearly in one direction.
- A vacuum leak, which can cause idle behavior to vary depending on engine temperature and load, producing an inconsistent rather than clearly-directional fault.
- A wiring or connector fault at the idle control valve, causing an intermittent rather than a consistent signal.
- A failing throttle position sensor, giving the control module unreliable data about the throttle’s actual position at idle.
How to Diagnose It
- Clean the idle air control valve and throttle body first — this resolves a large share of idle control codes regardless of which specific variant is stored.
- Check for any accompanying vacuum leak — listen for a hissing sound near the intake manifold with the engine running.
- Check the idle control valve’s wiring and connector for looseness or damage.
- Use live OBD-II data — see our OBD-II scanner guide — to watch idle RPM behavior directly and see whether it’s trending in one particular direction despite the unclassified code.
Never work under a vehicle supported only by a jack. Use axle stands on firm, level ground, chock the wheels that stay down, and leave the car in gear or Park with the handbrake on. A jack lifts; stands hold.
Possible Fixes
| Cause | Fix | Typical cost |
|---|---|---|
| Dirty IAC valve/throttle body | Clean the valve and throttle body | $0 – $100 |
| Vacuum leak | Replace the leaking hose/gasket | $20 – $300 |
| Wiring/connector fault | Repair or reseat the connector | $50 – $150 |
| Failing throttle position sensor | Replace the sensor | $100 – $250 |
Can I Keep Driving With This?
Yes, in most cases, but keep an eye on it — an unstable idle that occasionally stalls is more than an annoyance once it starts happening in traffic. See our car dies at idle guide if stalling has already started, and treat that combination as worth addressing sooner rather than later.
When to See a Mechanic
Not urgent if idle is merely rough but stable — try cleaning the throttle body first. Get it looked at sooner if the engine is actually stalling. More in our trouble codes section.
Frequently Asked Questions
What does P0505 mean?
It means the engine control module has detected a general idle control malfunction without being able to classify it as specifically too high or too low RPM.
What’s the difference between P0505 and P0506/P0507?
P0506 and P0507 identify a specific direction of fault (idle too low or too high), while P0505 is the general, unclassified version of the same underlying idle control problem.
What’s the most common cause of P0505?
A dirty or carbon-fouled idle air control valve or throttle body is the most common cause, the same leading cause across all three idle control code variants.
Is it safe to drive with a P0505 code?
Yes, generally, though a rough or unstable idle that occasionally stalls is worth addressing sooner rather than later, especially if it’s happening in traffic.
How much does it cost to fix P0505?
Often free to $100 for a simple throttle body cleaning, up to $300 if a vacuum leak or other component needs repair.
