DTC P0198 Engine Oil Temperature Sensor "A" Circuit High: Notes

DTC detecting condition: 

Immediately at fault recognition

Trouble symptom: 

• Hard to start
• Improper idling
• Poor driving performance

Wiring diagram: 

Engine Electrical System Ref. to WIRING SYSTEM>Engine Electrical System>WIRING DIAGRAM .

Fig 1: Engine Oil Temperature Sensor A Circuit High - Wiring Diagram

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK CURRENT DATA  .
   1. Start the engine.
   2. Using the Subaru Select Monitor, read the value in [Engine Oil Temperature].
   NOTE: For detailed procedures, refer to "Data Monitor". Ref. to ENGINE (DIAGNOSTICS) (H4DO)>Subaru Select Monitor .

   Is the value of [Engine Oil Temperature] -40°C (-40°F) or less?

   • Yes: Go to 2  .
   • No: Even if DTC is detected, the circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis.
     NOTE: In this case, temporary poor contact of connector, temporary open or short circuit of harness may be the cause.
2. CHECK FOR POOR CONTACT  .

   Check for poor contact between the ECM and engine oil temperature sensor connectors.

   Is there poor contact of the ECM or engine oil temperature sensor connectors?

   • Yes: Repair the poor contact of ECM or engine oil temperature sensor connector.
   • No: Go to 3  .
3. CHECK HARNESS BETWEEN ECM AND ENGINE OIL TEMPERATURE SENSOR CONNECTOR  .
   1. Turn the ignition switch to OFF.
   2. Disconnect the connectors from the engine oil temperature sensor.
   3. Disconnect the connector from ECM.
   4. Measure the resistance of the harness between the ECM connector and engine oil temperature sensor connector.

   Connector & terminal 

   (B135) No. 18 - (E75) No. 2:

   (B135) No. 34 - (E75) No. 1:

   Is the resistance less than 1 Ω?

   • Yes: Go to 4  .
   • No: repair the harness and connector.
     NOTE: In this case, repair the following item:

     • Open circuit in harness between ECM connector and engine oil temperature sensor connector
     • Poor contact of coupling connector
4. CHECK HARNESS BETWEEN ECM AND ENGINE OIL TEMPERATURE SENSOR CONNECTOR  .
   1. Connect all connectors.
   2. Turn the ignition switch to ON.
   3. Measure the voltage between ECM connector and chassis ground.

   Connector & terminal 

   (B135) No. 18 (+) - Chassis ground (-):

   Is the voltage 5 V or more?

   • Yes: Repair the short circuit to power in harness between ECM connector and engine oil temperature sensor connector.
   • No: Replace the engine oil temperature sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>Engine Oil Temperature Sensor .