P0505 Code: Idle Control System Malfunction
Quick Answer
The idle control system isn't maintaining the correct idle speed — the engine's idle is unstable or incorrect.
At a Glance
What this code means
The idle control system manages the engine's idle speed when your foot is off the accelerator. On older vehicles, this is done by an Idle Air Control (IAC) valve that varies bypass airflow. On modern drive-by-wire vehicles, the throttle plate itself handles idle control. P0505 means the ECU cannot achieve or maintain the target idle speed.
Common causes
- Dirty or stuck idle air control (IAC) valve
- Carbon buildup on the throttle body and idle bypass passage
- Vacuum leak allowing uncontrolled air into the engine
- Faulty throttle position sensor affecting idle target
- On drive-by-wire systems: faulty throttle body or pedal position sensor
Symptoms you might notice
- Rough or unstable idle
- Idle speed too low (engine stalls) or too high
- Hunting or surging idle
- Check engine light on
- Engine stalls when coming to a stop
Can you still drive?
With caution. If the engine is stalling at idle, it's dangerous in traffic.
How serious is this code?
Moderate. An engine that stalls at idle is a safety hazard. Address it promptly.
Before you replace parts
Parts replacement is often not the first step. Before buying anything, it's worth checking these basics:
- Clean the throttle body and IAC passage first — carbon buildup is the most common cause and costs almost nothing to fix
- Check for vacuum leaks before replacing any idle control components — a vacuum leak makes the idle uncontrollable regardless of how many parts are replaced
- Perform the manufacturer's idle relearn procedure after any throttle body cleaning before concluding a part is faulty
How to troubleshoot it
- Clean the throttle body and idle air passage — carbon buildup is the most common cause
- On IAC valve systems, remove and clean or replace the IAC valve
- Check for vacuum leaks — a vacuum leak raises idle speed above the ECU's commanded target
- Inspect the idle air control valve wiring and connector
- On drive-by-wire vehicles, perform a throttle body relearn procedure after cleaning
Tools that may help
These are the types of tools commonly used when diagnosing this code. Having the right tool can save time and help confirm a diagnosis before spending money on parts.
- OBD-II scanner with live idle RPM data
- Throttle body cleaner
- Multimeter
Estimated repair cost range
Throttle body cleaning (DIY): $10–$20 in cleaner. IAC valve replacement: $50–$200. Throttle body (drive-by-wire): $150–$400.
Costs vary significantly by vehicle, location, and whether you use OEM or aftermarket parts. These are general ranges for reference only.
When to call a mechanic
If you've completed the basic troubleshooting steps and the code keeps returning, or if you don't have access to the proper diagnostic tools, it's worth consulting a professional. A qualified technician can perform a full diagnosis and confirm the root cause before any parts are purchased. When in doubt, get a professional opinion — it can save you from an expensive misdiagnosis.
Real-world note
After cleaning the throttle body on a drive-by-wire vehicle, most vehicles require a throttle relearn procedure — disconnecting the battery or following the manufacturer's idle relearn steps. Skipping this step can cause idle problems after cleaning.
Informational purposes only. This guide is for educational reference and is not a substitute for diagnosis by a qualified technician. Repair costs, causes, and symptoms may vary by vehicle make, model, year, and condition. Always consult a licensed mechanic before performing major repairs.