DTC P0506: Idle Control System, Rpm Less Than Expected: Condition

WARNING: This page is about a different car, the 2002 Mazda 626. However, it is still accessible from the selected car via links, so may be relevant.

DTC is set when actual engine speed is 100 RPM less than programed target speed for 14 seconds with brake pedal depressed and front wheels in straight ahead position (power steering pressure switch is off). This is a continuous monitor. MIL is illuminated when PCM detects malfunction during 2 consecutive drive cycles. Possible causes are:

A/C relay control circuit malfunction.
Air filter or intake passage plugged.
Idle Air Control (IAC) valve malfunction.
Low engine compression or too much blow by.
Generator control circuit malfunction.
PCM malfunction.