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 keep the engine running while the car isn’t moving, the idle air control valve bypasses the closed throttle plate and continues to inject air into the engine. 

The idle air control 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 efficiently produce the required power to run major components like air conditioning, power steering, etc. 

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.

P0505 is a generic OBD-II code, so it may appear regardless of whether your vehicle is a Honda Accord, Ford Fusion, or something else.

P0505 Causes

A faulty idle control motor or carbon build-up in the IAC valve are the common causes of the P0505 engine 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

Issues with the MAF sensor

P0505 Symptoms

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

• Engine doesn’t idle properly unless the accelerator pedal is pressed
• Engine has a rough idle (possibly from a vacuum leak)
• Engine RPM is inconsistent from erratic idle speed
• Engine may stall while coming to a stop

Can I Still Drive With a P0505 Code?

DTC P0505 is a severe error code. 

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

With a P0505 engine code, you may experience inconsistent idle speed, rough idle, or worse — your vehicle may stall randomly at stop signs or in traffic. You may also notice reduced fuel economy over time. Moreover, driving around with this trouble code will further damage your car’s engine.

Diagnosing P0505

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 engine codes with an OBD II scan tool. 

• They may monitor the engine idle to check if it stays within the prescribed specifications. On older vehicles that use a throttle body with a manual throttle cable, they may use the idle screw to adjust the engine RPM.

• Then, your mechanic may check the air intake and Positive Crankcase Ventilation (PCV) system for a leak in a vacuum line 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 appears, 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 the PCM connector and all IAC wires. 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 engine light code P0505:

• They’ll start by clearing out all other engine codes.

• They may clean the throttle valve, body, and other components if they’re clogged with dirt and debris with a throttle body cleaner. 

Note: A throttle body cleaner is advisable instead of a carb cleaner. There’s a difference in ingredients, and carb cleaner is generally harsher. 

• They may fix any leaks in the vacuum line.

• They may repair the IAC wires or any PCM connector damages in case of open or short issues.

• If there’s an issue with the IAC, the mechanic may replace the idle control valve. They’ll disconnect the battery. After locating the IAC valve, they’ll unplug the IAC wiring and remove the old IAC valve. They’ll install the new idle control valve, reconnect the wiring, and finally 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 spare parts ($70 – $400) and labor charges ($50 – $100).