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, which controls the amount of air bypassing the throttle plate. If the 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 OFF, 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 ECT and TP sensor values. The PCM monitors the IAC circuit for low and high voltage faults. DTC P0508 sets when either coil A or coil B is commanded ON or OFF and the voltage on one side of the coil circuit is low when ignition voltage should be present.
DTC Parameters
DTC P0508 will set if either coil A or coil B is low, grounded, when it should be high, ignition voltage, when:
| • | The condition exists for longer than 0.5 seconds. |
| • | The PCM is commanding the IAC valve to move. |
A short to ground will set this DTC. The condition must exist when the PCM is commanding the IAC valve to move.
DTC P0508 diagnostic runs continuously when engine is running and the PCM is commanding the IAC valve to move.
DTC P0508 is a type D DTC.
Diagnostic Aids
To locate an intermittent problem, use a scan tool to monitor IAC MOTOR CKT. - LOW VOLT test while accelerating engine between idle and 3,500 RPM. When a fault, short to ground in any of the 4 circuits, shorted IAC coil 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 coils and 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. |
