P0505: unstable idle
The system that controls idle speed is not behaving as expected. Have it checked soon, especially if the engine sometimes stalls at idle.
Can you keep driving?
Yes, but be ready for it to stall when stationary.
What the code means
The engine may idle too high, too low, or unevenly. The car can tell it is not hitting the idle speed it is aiming for.
Common causes
- A dirty throttle body
- A component in the idle control on its way out
- A vacuum leak
What to do now
- Note when it happens: cold, warm, stationary, with the air conditioning on.
- If the engine stalls as you brake toward a red light, mention exactly that.
- Ask for the throttle body to be cleaned before parts are replaced.
What people often get wrong
Replacing parts by guesswork. Live data shows what the control is actually trying to do, and that shortens the diagnosis considerably.
When the car must go to a workshop
This can be diagnosed without much guessing once a technician sees live data from the scan tool. Ask for that.
Read next
This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.
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