DTC P0118: Engine Coolant Temperature Sensor 1 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 Refer to WIRING SYSTEM>ENGINE ELECTRICAL SYSTEM>WIRING DIAGRAM .

Courtesy of SUBARU OF AMERICA, INC.

1. CHECK CURRENT DATA. 
   1. Start the engine.
   2. Read the value of «Coolant Temp.» using the Subaru Select Monitor or a general scan tool.
   NOTE:

   • Subaru Select Monitor

     For detailed operation procedures, refer to "Current Data Display For Engine". Refer to ENGINE (DIAGNOSTICS) (EXCEPT FOR STI)>SUBARU SELECT MONITOR .

   • General scan tool

     For detailed operation procedures, refer to the general scan tool operation instructions.

   Is the value of «Coolant Temp.» less than -40°C (-40°F)?

   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 again.

   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 of ECM and engine coolant temperature sensor connector.

   Is there poor contact of ECM or engine coolant temperature sensor connector?

   Yes:  Repair the poor contact of ECM or engine coolant temperature sensor connector.

   No:  Go to  3.

3. CHECK HARNESS BETWEEN ECM AND ENGINE COOLANT TEMPERATURE SENSOR CONNECTOR. 
   1. Turn the ignition switch to OFF.
   2. Disconnect the connector from ECM.
   3. Disconnect the connectors from the engine coolant temperature sensor.
   4. Measure the resistance of harness between ECM connector and engine coolant temperature sensor connector.

   Connector & terminal 

   (E159) No. 27 - (E8) No. 1:

   (E158) No. 38 - (E8) No. 2:

   Is the resistance less than 1 Ω?

   Yes:  Go to  4.

   No:  Repair the open circuit of the harness between the ECM connector and engine coolant temperature sensor connector.

4. CHECK HARNESS BETWEEN ECM AND ENGINE COOLANT TEMPERATURE SENSOR CONNECTOR. 
   1. Connect all connectors.
   2. Turn the ignition switch to ON.
   3. Measure the voltage between ECM connector and engine ground.

   Connector & terminal 

   (E158) No. 38 (+) - Engine ground (-):

   Is the voltage 5 V or more?

   Yes:  Repair the short circuit of harness to power supply between ECM connector and engine coolant temperature sensor connector.

   No:  Replace the engine coolant temperature sensor. Refer to FUEL INJECTION (FUEL SYSTEMS) (EXCEPT FOR STI)>ENGINE COOLANT TEMPERATURE SENSOR .