DTC P0505 - Idle Control System Malfunction: Condition

WARNING: This page is about a different car, the 1997 Mazda Millenia. However, it is still accessible from the selected car via links, so may be relevant.

DTC is set when actual engine speed is excessively higher or lower than target speed preset in ECM for more than 14 seconds. Possible causes are:

Idle Air Control (IAC) valve malfunction.
Leakage in intake air system.
Open or short circuit between terminal "A" on IAC valve and main relay.
Open or short circuit between terminal "B" on IAC valve and terminal No. 4M on Engine Control Module (ECM).