Is It Safe to Drive With a P0505 Code?
Common Symptoms of a P0505 Fault
Rough or fluctuating idle
Stalling at idle, especially when cold
RPM hunting at standstill
EML on
Difficult starting
Possible Causes of P0505
A P0505 code can be triggered by faults in several different vehicle systems. Identifying the specific root cause requires proper diagnostic equipment and live data analysis — not just reading the code itself.
IAC Valve Carbon Build-up
Carbon deposits on the IAC valve or throttle body preventing correct airflow.
IAC Valve Wiring Fault
Electrical fault in the IAC valve circuit.
IAC Valve Failure
The valve itself has failed mechanically or electrically.
Vacuum Leak
An air leak at idle causing the IAC system to over-compensate.
Why we don't list the exact part: A P0505 code tells you what's happening but not why. Replacing parts based on the code alone without proper diagnosis is the most common way drivers waste money. A proper diagnosis identifies the exact cause first.
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P0505 and Your MOT
A stored P0505 fault code will cause your car to fail its MOT. The engine management light will be illuminated during the test, which is an automatic failure under current MOT testing standards. Even if the light clears temporarily, the code remains in the ECU memory and can be read by the tester's equipment.
Get the fault properly diagnosed and repaired first — then have the code cleared and confirm the light doesn't return before booking your test.