Mazda Miata P0505: Idle Air Control System - Malfunction

P0505 An OBDII Code that addresses the idle air control system within the Mazda Miata.  It’s a generic code, which means that it has the same meaning for the Miata as it would any other vehicle (Miata or not).

P0505:  Idle Air Control System:  Malfunction

P0505 triggers when there is either a stuck or clogged idle control valve or a vacuum leak.  However, there are other possible causes.  Although, they aren’t as prevalent.

P0505 Mazda Miata


Índice de Contenido
  1. P0505 Symptoms:  Mazda Miata
  2. Miata P0505 Causes
    1. Valve to control the idle airflow
    2. Vacuum Leak
    3. Carbon Buildup
  3. Conclusion

P0505 Symptoms:  Mazda Miata

This particular trouble code lets you know that there is something wrong with the idle air control system, but that’s about it.  The symptoms may vary.

Below are some symptoms that P0505 is most commonly seen:

  • Soon, Service engine light
  • It is a rough idle
  • Stalling
  • Low idle speed 


Miata P0505 Causes

P0505 can usually be caused either by a vacuum leak or an issue with the idle control valve.  While replacing the idle air control valve seems like a good idea, it’s worth your time to check for a Leakage of vacuum first.

Mazda Miata P0505 Diagnosis

Valve to control the idle airflow

The idle air control valve’s primary job is to regulate the idle speed of your engine.  The idle air control valve regulates the engine's idle speed by controlling the amount of air that is allowed in.

It is possible for the IAC to become stuck or dirty.  The IAC valve may require a replacement or cleaning may suffice.  Here’s a great You Tube video How to clean an idle air control valve  

They are available if you decide that an idle valve control valve is needed. It is very affordable.


Vacuum Leak

While the idle air control valve is often the cause of P0505, it’s a great idea to start diagnosing it by looking for a vacuum leak.  Check the vacuum tubes surrounding the IAC.  It may be necessary to replace them if they appear cracked or dry-rotted.

Leakages around the throttle body and intake manifold can cause your Miata code to be thrown.  Here’s a great article from Popular Mechanics on How to locate a vacuum leak.


Carbon Buildup

Carbon buildup can occur around the IAC's inlet.  The problem can be fixed by inspecting the IAC area for carbon-blocking passages.

It can cause the intake of air to be reduced if the throttle body becomes too dirty.  This can cause P0505 to be thrown.  Often, when this is the case, you’ll feel the engine stall for a second before it recovers when slowing down.  

This happens because the engine can’t adjust to this problem fast enough.  It is possible to fix the problem by cleaning it from around the throttle blades.



You will be able to diagnose P0505 on your Miata.  Most of the time it’s a vacuum leak or the idle air control valve.  If you feel like there’s anything that you can add, please feel free to leave a comment below.

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