For 1990-2009 cars

The idle speed on DOHC and SOHC 1.9L engines is controlled by the 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, which controls the amount of air bypassing the throttle plate. If the powertrain control module (PCM) wants the IAC valve to extend, it will pulse battery voltage to each of the coils, and flip polarity, in a certain sequence, so both coils will be energized at certain times during pintle extension. The PCM will reverse the sequence during retraction. The IAC position is measured in counts, which can be read on the scan tool. When the engine is turned OF, the IAC valve is retracted 80-100 counts allowing for maximum air during a restart. After the engine is started, the motor position command is based on engine coolant temperature (ECT) and throttle position (TP) sensor values. The PCM monitors the IAC circuit for low and high voltage faults. DTC P0509 sets when either coil A or coil B is commanded ON or OFF and the voltage on one side of the coil circuit is high when it should be grounded.

### DTC Parameters

DTC P0509 will set if either coil A or coil B is high, ignition voltage, when it should be low, grounded, when:

• | The condition exists for longer than 0.5 seconds.

• | The PCM is commanding IAC valve to move.

An open or short to voltage will set this DTC. The condition must exist when the PCM is commanding the IAC valve to move.

DTC P0509 diagnostic runs continuously when engine is running and the PCM is commanding the IAC valve to move.

DTC P0509 is a type D DTC.

### Diagnostic Aids

To locate an intermittent problem, use a scan tool to monitor IAC MOTOR CKT. - OPEN or IAC MOTOR CKT. - HIGH VOLT test while accelerating engine between idle and 3,500 RPM. When a fault, open/short to voltage on any of the four circuits or open PCM driver exists, the scan tool will display TEST FAIL. The test updates every 100 ms when the IAC valve is commanded to move.

Moving the valve in one direction with the scan tool will allow current to flow through both the coils and the polarity to flip on both as well. Due to this design, both DTC P0508 and P0509 will run when the IAC valve is moved in only one direction.

IAC terminal resistance:

• | A to B: 40-80 ohms

• | C to D: 40-80 ohms

• | All other combinations should be open