The idle speed on DOHC and SOHC 1.9L engines is controlled by the powertrain control module (PCM) via an idle air control (IAC) valve. The IAC valve consists of a 2 coil stepper motor that moves a pintle valve in and out from the seat controlling the amount of air bypassing the throttle plate. The PCM controls the IAC valve by supplying ignition voltage or ground to any of the four circuits of coil A or coil B. When one circuit of a coil is supplied ignition voltage, the other side is supplied ground. Coil A and coil B are independent of each other, however both must be functional in order for the valve to move correctly. The IAC position is measured in counts, which can be read on the Scan tool. When the engine is turned OFF, the IAC is retracted 80-100 counts allowing for maximum air during a restart. After the engine is started, the motor position command is based on ECT and throttle position (TP) sensor values. DTC P0506 sets when the actual engine speed is 100 RPM or more below the desired idle speed.

DTC P0506 will set if actual engine speed is 100 RPM or more below the desired idle speed when:

DTC P0506 diagnostic runs continuously every 17 seconds when the above conditions have been met.

DTC P0506 is a type B DTC.

Important: If DTC P0508 or P0509 is set, diagnose that DTC first. A diagnosed IAC valve circuit may have caused this DTC to set.

To locate an intermittent problem, use a scan tool to command different idle speed RPM (650-875). ENGINE SPEED should match the DESIRED IDLE SPEED within 100 RPM.

IAC terminal resistance:

Use the following flowchart for possible causes of low idle speed.