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Home >> Subaru >> 2002 >> Forester Base >> Repair and Diagnosis >> Engine Performance >> Testing & Diagnosis >> Engine Controls - Self-Diagnostics - 2.5L >> Diagnostic Tests >> DTC P0037: Rear Heated Oxygen Sensor (Ho2S) Heater Circuit Malfunction (Forester, IMPREZA & Outback Sport)

DTC P0037: Rear Heated Oxygen Sensor (Ho2S) Heater Circuit Malfunction (Forester, IMPREZA & Outback Sport)

  1. Perform test procedure. See TESTING PROCEDURE . Turn ignition off. Disconnect 35-pin ECM electrical harness connector B134. Using a DVOM, measure resistance of Black/Yellow wire between 35-pin ECM electrical harness connector B134 terminal No. 35 and good chassis ground point. See Figure. If resistance is less than 5 ohms, go to step  3. If resistance is not less than 5 ohms, go to next step.
  2. Locate and repair open circuit in ground circuit and check for proper terminal contact in all connectors between ECM and engine ground point located on rear of engine. Then, use a DVOM to measure resistance of Black/Yellow wire between 35-pin ECM electrical harness connector B134 terminal No. 34 and good chassis ground point. See Figure. If resistance is less than 5 ohms, go to next step. If resistance is not less than 5 ohms, locate and repair open circuit in ground circuit and check for proper terminal contact in all connectors between ECM and engine ground point located on rear of engine. See ENGINE PERFORMANCE in appropriate SYSTEM WIRING DIAGRAMS article in ELECTRICAL. Clear memory and perform verification procedure. See CLEARING DIAGNOSTIC TROUBLE CODES  and VERIFICATION PROCEDURE  .
  3. Start engine. Using a scan tool, observe current value for rear HO2S heater. If current draw is not more than 0.2 amp, go to next step. If current draw is more than 0.2 amp, check for poor contact in connectors at ECM and rear HO2S. See ENGINE PERFORMANCE in appropriate SYSTEM WIRING DIAGRAMS article in ELECTRICAL. Clear memory and perform verification procedure. See CLEARING DIAGNOSTIC TROUBLE CODES  and VERIFICATION PROCEDURE  .
  4. Start and idle engine. Using a DVOM, measure voltage of Brown/White wire by backprobing between 35-pin ECM electrical harness connector B134 terminal No. 21 and good chassis ground point. See Figure. If voltage is less than one volt, go to step  7. If voltage is not less than one volt, go to next step.
  5. Turn ignition on. Using a DVOM, measure voltage of Brown/White wire by backprobing between 35-pin ECM electrical harness connector B134 terminal No. 21 and good chassis ground point while wiggling harness and connector at ECM. See Figure. If voltage does not change less than one volt while wiggling harness and connector, go to next step. If voltage change is less than one volt while wiggling harness and connector, check for poor terminal contact at ECM connector. Clear memory and perform verification procedure. See CLEARING DIAGNOSTIC TROUBLE CODES  and VERIFICATION PROCEDURE  .
  6. Turn ignition off. Disconnect rear HO2S 4-pin electrical harness connector. Using a DVOM, measure voltage of Brown/White wire by backprobing between 35-pin ECM electrical harness connector B134 terminal No. 21 and good chassis ground point. See Figure. If voltage is less than one volt, replace ECM. If voltage is not less than one volt, locate and repair short to voltage in harness between ECM and rear HO2S. Afterward, check for damage to ECM and replace ECM as required. See ENGINE PERFORMANCE in appropriate SYSTEM WIRING DIAGRAMS article in ELECTRICAL. Clear memory and perform verification procedure. See CLEARING DIAGNOSTIC TROUBLE CODES  and VERIFICATION PROCEDURE  .
  7. Turn ignition off. Disconnect rear HO2S 4-pin electrical harness connector. Turn ignition on. Using a DVOM, measure voltage of Yellow/Red wire between harness side of 4-pin rear HO2S electrical harness connector terminal No. 2 and good chassis ground point. See Fig 1. If voltage is more than 10 volts, go to next step. If voltage is not more than 10 volts, locate and repair open circuit in power supply line and check for proper terminal contact in connectors between rear HO2S and main relay. Main relay is located under right side of dash (left side of dash on Forester). See ENGINE PERFORMANCE in appropriate SYSTEM WIRING DIAGRAMS article in ELECTRICAL. Clear memory and perform verification procedure. See CLEARING DIAGNOSTIC TROUBLE CODES  and VERIFICATION PROCEDURE  .
  8. Turn ignition off. Disconnect rear HO2S 4-pin electrical harness connector. Using a DVOM, measure resistance between rear HO2S 4-pin electrical harness connector terminals No. 1 and 2. See Fig 2. If resistance is less than 30 ohms, locate and repair open circuit in harness and check for proper terminal contact in connectors between rear HO2S and ECM. See ENGINE PERFORMANCE in appropriate SYSTEM WIRING DIAGRAMS article in ELECTRICAL. If resistance is not less than 30 ohms, replace rear HO2S. Clear memory and perform verification procedure. See CLEARING DIAGNOSTIC TROUBLE CODES  and VERIFICATION PROCEDURE  .
Fig 1: Identifying Terminals On Harness Side Of 4-Pin Rear HO2S Connector
G00013011Courtesy of SUBARU OF AMERICA, INC.
Fig 2: Identifying Rear HO2S Terminals
G00013027Courtesy of SUBARU OF AMERICA, INC.
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