P0505: Idle Control System Malfunction

What Is P0505?

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

The idle air control system controls the engine speed when the vehicle is idle. It uses an Idle Air Control valve (IAC valve), typically mounted on or near the throttle body, to regulate airflow through dedicated IAC passages within the throttle body.

When the throttle plate is closed, such as during accelerator release or while the vehicle is stationary, the air supply to the engine is restricted. In order to run the engine under these conditions, the idle air control valve manages the engine idle speed by increasing and decreasing the airflow as needed. 

When more power is needed, the Powertrain Control Module (PCM) commands the idle air control system to adjust the airflow into the engine. This helps the engine produce the required power to run major components like air conditioning and power steering. 

The PCM constantly monitors the airflow by checking voltage signals in the IAC circuit. It triggers the P0505 trouble code if it notices unusual voltage readings, indicating an IAC issue.

Now, P0505 is a generic OBD-II code, so it may appear regardless of whether your vehicle is a Honda Accord, Dodge, or any other car model.

But here’s the thing:
Other issues, such as a faulty Throttle Position Sensor (TPS), clogged air filter, and bad Mass Airflow Sensor (MAF sensor), can also affect airflow and trigger the error code P0505. So, it’s best to have your vehicle diagnosed by a professional.

NOTE: The IAC valve (also called IAC motor) or valve is usually used in cars with multiport electronic fuel injection systems.

P0505 Symptoms

Most symptoms may be noticeable only when your vehicle is idle. Aside from a lit check engine light (CEL), here are other DTC P0505 signs that’ll help you identify an IAC problem:

1. Engine doesn’t idle properly unless the accelerator pedal is pressed
2. Engine has a rough idle (possibly from a vacuum leak)
3. Engine RPM is inconsistent from erratic idle speed
4. Engine may stall before coming to a stop

P0505 Causes

Here are some reasons why code P0505 may occur:

1. Faulty idle air control motor
2. Carbon build-up in the IAC valve
3. IAC passages are clogged with dirt, debris, etc.
4. Damaged idle air control motor connector
5. Open or shorted IAC circuit
6. Dirty throttle body, including throttle plate
7. Malfunctioning MAF sensor or TPS
8. Vacuum leak in the intake manifold system
9. Clogged air filter
10. Leaking Powertrain Crankcase Ventilation valve (PCV valve)
11. Issues with PCM, such as software update

Can You Still Drive With a P0505 Code?

No, DTC P0505 is a severe error code that needs immediate attention. An idle control system malfunction can significantly affect how your car runs, making it unsafe to operate.

You may experience a misfire, rough idle, or difficulty maintaining consistent idle speed, especially during a cold start. Even worse, your vehicle may stall randomly at stop signs or in traffic.

You may also notice reduced fuel economy and increased emissions over time. Ignoring erratic idle can stress out the engine components, leading to expensive repair costs, like throttle body or catalytic converter replacement.

That’s why it’s best to have a mechanic address an illuminated check engine light and P0505 code before driving your vehicle.

How to Diagnose Code P0505

No matter how much of a DIY enthusiast you are, diagnosing the P0505 code without proper technical knowledge can lead to incorrect repairs or replacements. Instead, getting a trusted mechanic to resolve the IAC issue accurately can save you from unnecessary risks and ensure the system’s continuity.

Here’s how your mechanic will diagnose the P0505 trouble code:

1. Scan the car for engine codes with an OBD II scan tool. 

2. Examine if the engine idle stays within the prescribed spec mentioned in the owner’s manual. On older vehicles that use a throttle body with a manual throttle cable, they may use the idle screw to adjust the engine RPM.

3. Check the air intake and PCV valve for a leak in a vacuum line with a smoke machine. They may also check for leaks in the intake manifold system.

4. Examine the MAF sensor for any issues.

5. Inspect the throttle plate and IAC valve inlet for carbon build-up.

6. Inspect IAC wires and connectors for any damage.

7. Disconnect the IAC motor to check if the diagnostic trouble code reappears.

If code P0505 still persists, there may be a problem with the PCM. The mechanic will monitor for continuity in the PCM connector and all IAC wires (depending on the valve configuration). They will disconnect every IAC wire to check if the code reappears.

Possible Repairs & Costs

P0505 code may arise from a software glitch, mechanical issue, or technical malfunction. 

Here’s what a mechanic may do to resolve engine light code P0505:

1. Clear out all other engine codes.

2. Clean the throttle body, valve, and other components with a throttle body cleaner if they’re clogged with dirt and debris. 

NOTE: A throttle body cleaner is advisable instead of a carb cleaner since the latter is generally harsher. 

3. Fix any leaks in the vacuum line.

4. Repair the IAC wires or any PCM connector damages in case of an open or short wire.

5. If the IAC valve is faulty, the mechanic may replace it. Here’s how:

• They’ll first disconnect the negative terminal of the battery, followed by the positive one. 
• The mechanic will locate the IAC valve, unplug the IAC wiring, and remove the old IAC valve. 
• They’ll then install the new IAC valve, reattach the wiring, and reconnect the battery terminals in reverse order — positive first, then negative. 

Replacement or repair costs depend on the car make and model, the parts used (aftermarket or OEM parts), the difficulty of the repair, and local labor charges. 

Here’s how much these processes can cost you:

• Vacuum leak repair: $100 to $1,500
• New IAC valve replacement: $120 to $500
• Throttle body cleaning: About $150
• PCV Valve Replacement: $180 to $220
• MAF sensor replacement: $450 to $530

6 Tips to Maintain the IAC System

Here’s how you can optimize your IAC system’s lifespan:

1. Clean the IAC valve routinely
2. Replace the air filter regularly
3. Perform regular ECU diagnostics
4. Keep the throttle body clean
5. Avoid excessive idling
6. Stick to your vehicle’s maintenance schedule