P0505 fault code: Idle Air Control System
We have no verified driving guidance for P0505. Treat as do not drive until checked. Stop at once if the engine stalls while moving or idle will not hold.
Can I DIY?
Professional-only boundary. You may confirm P0505 and observe idle in Park; do not remove idle-air control parts yourself.
This is the conservative boundary from the reviewed code record. This page has no frozen site-wide safety grade, so it does not publish a repair route.
What to tell the garage about P0505
Stop driving now. The engine computer has flagged a fault in the idle air control system, which meters air when you are stopped or crawling. The code names that function, not a part you should order from this page.
Take these reviewed, code-specific details with you. They help the garage confirm the fault without turning this page into a repair procedure.
- Idle Air Control System: Registration number
- Idle Air Control System: Exact code P0505 and any companion codes
- Idle Air Control System: Whether the engine management light is steady or flashing
- Idle Air Control System: Whether idle hunted, stayed high or low, or the engine stalled
Never idle in a closed garage to read codes — an open door is not enough. CO is odourless and can kill. Run diagnostics in open air. Headache or nausea with exhaust smell: get fresh air and stop.
What does P0505 mean?
P0505 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P05xx block, which covers vehicle speed and idle control. The stored description is “Idle Air Control System”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.
Will P0505 fail an MOT?
If the malfunction indicator lamp is illuminated when P0505 is stored, DVSA treats that as a Major defect (test fail) until the fault is repaired and the lamp remains off. Idle quality is not itself an MOT pass; an illuminated malfunction lamp still fails the test.
Before booking your test, check what usually fails on your car: see our MOT pass rates by make and model, built from 33.1 million DVSA tests.
Related guides
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P0505 — quick answers
Is it safe to drive with P0505?
Stop driving now. We have no verified driving guidance for P0505. Treat as do not drive until checked. Stop at once if the engine stalls while moving or idle will not hold.
Will P0505 fail my MOT?
If the malfunction indicator lamp is illuminated when P0505 is stored, DVSA treats that as a Major defect (test fail) until the fault is repaired and the lamp remains off. Idle quality is not itself an MOT pass; an illuminated malfunction lamp still fails the test.
Fault-code identifiers are standard diagnostic facts. Their explanations and diagnostic guidance on this site are original editorial summaries, not reproduced SAE wording. MOT defect rules reflect the DVSA MOT inspection manual (2026). This page is general guidance for UK drivers, not a substitute for a professional diagnosis.