P0505 Code: Idle Control System Malfunction

What the P0505 code means

With your foot off the gas, the engine needs a minimum amount of air, perfectly controlled, to keep running. That airflow is regulated in one of two ways depending on the vehicle: with an idle air control valve that passes air through a channel parallel to the throttle plate, or directly with the electronic throttle body, which opens the throttle plate a few degrees.

The computer sets an rpm target based on temperature and the loads that are on, acts on that system and checks the result.

The P0505 code is stored when the actual idle does not match the commanded idle, either above or below it. The detail worth knowing is about adaptations: the computer memorizes how far it has to open to get the right idle, and that memory is adjusted over the years as carbon narrows the passage. When you clean the throttle body, that memory is out of date and the engine can be left with a high or unstable idle until the adaptations are reset. This is the cause of many complaints right after a cleaning that looked like it went fine.

It also helps to know that idle is not a fixed value. The computer raises it on purpose when the engine is cold, when the A/C is turned on, when you turn the power steering, or when the alternator has to charge the battery. A higher idle than usual under those conditions is not a fault but expected behavior, and mistaking it leads to unnecessary diagnosis.

P0505 code symptoms

• The engine stalls when you stop or lift off the gas.
• Hunting idle that rises and falls in a cycle.
• Idle that stays too high all the time.
• Hard starting that needs you to press the gas pedal.
• Rpm drop when the A/C is switched on or the steering wheel is turned.
• Check engine light on, with the P0505 code stored.
• Higher fuel use if the idle stays elevated.
• Idle that does not settle after a battery change or a long disconnect.

Likely causes of the P0505 code

1. Carbon buildup in the throttle body, which narrows the airflow. The dominant cause.
2. Dirty or sticking idle air control valve on systems that have one.
3. Adaptations not reset after a cleaning, which leaves the idle off.
4. Vacuum leaks, which let in uncontrolled air and upset the idle.
5. PCV valve stuck open.
6. Canister purge valve that does not close.
7. Throttle position sensor out of range.
8. Electronic throttle body with a failed motor or position sensor.
9. Damaged actuator wiring or connector.
10. Misfires or a wrong air-fuel mixture, which keep the engine from holding a stable idle for reasons unrelated to this system.
11. Battery disconnected without running the idle relearn again, which erases the stored adaptations.

How to diagnose the P0505 code step by step

1. Compare commanded rpm with actual rpm in live data. That is the reading that confirms whether the system cannot reach its target.
2. Remove and inspect the throttle body. Carbon around the throttle plate is usually obvious to the eye.
3. Check the throttle opening percentage at idle. A very high value means the system is making up for a restriction or a leak.
4. Look for vacuum leaks with smoke, including the brake booster hoses and the crankcase ventilation system.
5. Disconnect the canister purge valve and see whether the idle settles.
6. Verify the throttle position sensor signal through its whole travel, looking for jumps or dead spots.
7. Check the actuator power and ground with the multimeter.
8. Rule out misfires or mixture codes, which cause an unstable idle even when this system is fine.
9. Rule out legitimate idle increases. A cold engine, the A/C, power steering and alternator load all raise it on purpose.

How to fix the P0505 code

• Clean the throttle body with the proper cleaner and a soft rag, without scratching the throttle plate or forcing it by hand on electronic bodies.
• Reset the idle adaptations with the scan tool after cleaning. Without this step the engine can be left with a high or unstable idle even after a correct cleaning.
• Clean or replace the idle air control valve on systems that have one.
• Repair the vacuum leaks you find and replace the PCV valve if it is stuck.
• Replace the canister purge valve if it does not close.
• Replace the throttle body when its motor or position sensor has failed.
• Correct any misfire or mixture fault first, because they prevent a stable idle on their own.
• Run the idle relearn procedure after you disconnect or replace the battery.

Technical reference data

• Typical warm idle: 600 to 850 rpm depending on the engine.
• Cold idle: 1000 to 1500 rpm, dropping as the engine warms up.
• Throttle opening at warm idle: usually 2 to 8 percent.
• Throttle position sensor signal: from 0.5 volt at rest to 4.5 volts at full throttle, with no jumps.
• After cleaning the throttle body you need to reset the adaptations with the scan tool.
• The system raises idle on purpose with a cold engine, A/C, power steering and alternator load.

Brand-specific notes

On electronic throttle bodies you must not move the throttle plate by hand or force it during cleaning: the internal mechanism is delicate and can be thrown out of calibration. Clean with the right product, and do not take the assembly apart more than you need to.

The step people forget most is the adaptation reset. An engine that is left with a high idle right after a correct cleaning almost always needs only that.

Frequently asked questions about P0505