P0505Powertrain
Idle air control system malfunction
Where to start
The idle air control valve. On engines with a cable throttle it is usually enough to take it off and wash it — the hunting idle goes away with the carbon. On a drive-by-wire throttle the body itself is cleaned, and re-learning afterwards is mandatory, otherwise it gets worse.
A code is not a diagnosis. It says which check failed, not which part to replace. Clearing it without repair only means the car will report it again in a few trips.
How to put together a report a mechanic will actually read
Step by stepUnstable idle — how to find the cause
How the code is built
Letter — the systemPowertrain: engine and transmission
Common or the maker's ownGeneric code: the same on every car
Which part of the engineIdle and auxiliary systems
Number of the faultThe serial number inside that group — it does not say how serious the fault is
All codes in «Idle and auxiliary systems»
How it feels behind the wheel
The same fault shows itself in different ways. Each sign below says what to look at in the live data.
Idle speed huntsThe tachometer needle wanders up and down with the car standing still: 1200, then 600. Sometimes the engine nearly stalls and picks up again.It stalls at idleAt the lights, or when you lift off, the engine dies. It usually restarts straight away.
How the app helps
CarScan reads this code from your car through an OBD-II adapter, shows whether it is active now or only stored, names the module that reported it, and puts it into a plain-text report for your mechanic.