What P0505 means
This code means the computer noticed the engine's idle speed is not doing what it expects. Every engine has a way of controlling how much air sneaks past the closed throttle when your foot is off the gas, to keep idle speed steady whether the A/C is running, the engine is cold, or you just popped it into gear. P0505 sets when that system is not behaving the way the computer commands it to, or the computer can't tell if it's working at all. It's a broad code. It does not point at one specific part.
Is it safe to drive
Usually yes, for a while. Common symptoms are idle that hunts up and down, idle that's too high or too low, stalling at stop signs, or rough idle when the A/C or power steering kicks in. If the car stalls in traffic or won't idle at all, that's worth getting looked at sooner rather than later. If it's just a slightly rough or slightly high idle, you can drive it, but keep an eye out for stalling getting more frequent, since that's the part that turns into an actual safety issue.
What commonly causes it
Roughly in order of how often we see it:
- A dirty throttle body or idle air passage, especially on older port-injected engines. Carbon buildup restricts airflow and confuses the system.
- A vacuum leak somewhere in the intake — a cracked hose, a bad gasket, a loose connection. This is a big one and it's not the idle control part's fault at all.
- The idle air control valve or motor itself, if the engine has one as a separate part. Some engines use an electronic throttle body instead and don't have this part.
- A dirty or failing throttle position sensor, or on drive-by-wire cars, a throttle body that needs a relearn procedure after cleaning or a battery disconnect.
- Wiring or connector problems at the idle control component.
What causes this varies by engine. Older cars with a separate idle air control valve and a cable throttle are a different animal from newer cars with electronic throttle control, where the throttle body itself manages idle and there's no separate valve to blame.
Diagnosis and rough cost
Diagnosis starts with pulling live data to see what the idle control system is actually doing versus what the computer wants, then checking for vacuum leaks and looking at the throttle body and related sensors before anything gets replaced. That's the part that keeps you from buying a part you don't need — a fifteen-dollar hose can cause the same code as a bad throttle body.
Cost depends entirely on what's found. A throttle body cleaning is a relatively minor job. A vacuum leak fix depends on where it is. Replacing an idle air control valve or a throttle body assembly costs more, and electronic throttle bodies run higher than older cable-operated ones. We'd rather tell you what we find before quoting a number than guess at it here.