P0505: Idle Control System Malfunction

What is P0505?

Diagnostic trouble code P0505 is defined as “Idle Control System Malfunction.”

The idle control system controls the engine speed when the vehicle is idle. It does so through an Idle Air Control valve (IAC valve) located at the IAC passages inside the engine throttle body.

Note: The IAC motor or valve is usually used in cars with Multiport Electronic Fuel Injection Systems. 

The throttle plate closes when a vehicle is idle (or when you release the accelerator pedal.) This limits the air supply to the engine. To allow the engine to run smoothly when the vehicle isn’t moving, the IAC valve bypasses the closed throttle plate, and continues to inject air into the engine. 

The IAC valve manages the engine idle speed by increasing and decreasing the airflow as required. 

When more power is needed, the powertrain control module (PCM) commands the idle control system to adjust the airflow into the engine. This helps the engine produce the required power to run major components like the air conditioning, power steering, etc., efficiently. 

The powertrain control module monitors the airflow through voltage variations in the IAC circuit. It sets code P0505 when it detects an irregular voltage in any IAC circuit.

Common symptoms

Most symptoms may be noticeable only when your vehicle is idle. Apart from an illuminated Check Engine Light, the common DTC P0505 signs that’ll help you diagnose an IAC malfunction are:

• Engine doesn’t idle properly unless you press the accelerator pedal
• Your vehicle may have a rough idle (due to an engine vacuum leak)
• Engine may die while coming to a stop
• Erratic idle speed

Can I still drive?

DTC P0505 is a severe code. 

Once an illuminated Check Engine Light and P0505 code show up, it’s best to get a mechanic to resolve the issue before driving your vehicle.

With a DTC P0505, your vehicle may stall randomly at stop signs or in traffic. You may also notice your fuel economy worsening over time. Moreover, driving around with this trouble code will further damage your car’s engine.

P0505 causes

A faulty idle control motor or carbon build-up in the IAC are the common causes of the P0505 code. Here are some more reasons why code P0505 may occur:

• Damaged idle air control motor connector
• Carbon build-up in the throttle body
• Vacuum leak in the intake system
• IAC passages are clogged with dirt, debris, etc
• Open or shorted IAC circuit

Diagnosis

If you misdiagnose error code P0505, you may replace or repair unnecessary parts to resolve this error code. It’s best to have an expert look at your vehicle when you spot an issue. 

Your mechanic may follow the below steps to diagnose the P0505 trouble code:

• The mechanic may scan your car for any other error code with an OBD II scan tool.
• They may monitor the engine idle to check if it stays within the prescribed specifications.
• Then, your mechanic may check the air intake and Positive Crankcase Ventilation (PCV) system for a vacuum leak with a smoke machine. 
• Once done, they may inspect the throttle plate and IAC valve inlet for carbon build-up.
• If the P0505 trouble code still shows up, they may disconnect the IAC motor to check if the code reappears.
• If it does return, they’ll check IAC wires and connectors for any damage.
• Finally, If code P0505 still persists, there may be issues with the powertrain control module. The mechanic may monitor for continuity in all IAC wires and PCM connectors. They may disconnect the wires to check if the code reappears.

Possible repairs for P0505 & Costs

The P0505 code may be due to software, mechanical, and technical issues. Here’s what a mechanic may do to resolve error code P0505:

• Your mechanic may start by clearing out all other error codes.
• They may clean the throttle valve, body, and other components if it’s clogged with dirt and debris using a carb cleaner or a throttle body cleaner.
• The mechanic may repair IAC wires and connectors in case of open or short issues.
• If there’s an issue with the IAC, your mechanic may replace the IAC valve. They’ll disconnect the battery to replace the idle control valve. After locating the valve, the mechanic will unplug the IAC wiring and remove the old IAC valve. Finally, they’ll install the new idle control valve, the wiring harness, and reconnect the battery.

The cost of an IAC valve replacement depends on your car’s make, model, and local labor rates. Usually, it may cost around $120 to $500. This includes both spare parts ($70 – $400) and labor charges ($50 – $100).