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Home › Subaru › 2003 › Forester X, Automatic › Repair and Diagnosis › Engine Performance › System › Engine Controls - Self-Diagnostics - 2.5L › Diagnostic Tests › DTC P0452: EVAP System Pressure Sensor Low Input › Diagnostic Procedure

Diagnostic Procedure

  1. Perform basic diagnostic procedure. See BASIC DIAGNOSTIC PROCEDURE . Turn ignition off. Remove and reinstall fuel filler cap. Turn ignition on. Using scan tool, observe fuel tank pressure. If pressure is less than -0.83 in. Hg, go to next step. If pressure is not less than -0.83 in. Hg, system has returned to normal. Check for intermittent open or short in wiring or poor terminal contact in connectors between fuel tank pressure sensor and ECM. See ENGINE PERFORMANCE in appropriate SYSTEM WIRING DIAGRAMS article in ELECTRICAL.
  2. Using a DVOM connected to ground, backprobe ECM 28-pin connector B135 (Baja, Legacy and Outback) or B134 (Forester and Impreza), terminal No. 3. See Figure. If voltage is more than 4.5 volts, go to step  4. If voltage is not more than 4.5 volts, go to next step.
  3. Using a DVOM connected to ground, backprobe ECM 28-pin connector, terminal No. 3 and wiggle harness and ECM connector. If voltage changes more than 4.5 volts while wiggling harness and ECM connector, locate and repair open circuit in wiring and check for poor terminal contact in connectors between ECM and fuel tank pressure sensor. If voltage does not change more than 4.5 volts while wiggling harness and ECM connector, contact authorized Subaru dealership because probable cause is deterioration of multiple parts.
  4. Using a DVOM connected to ground, backprobe ECM 28-pin connector, terminal No. 26. If voltage is less than 0.2 volt, go to step  6. If voltage is not less than 0.2 volt, go to next step.
  5. Using scan tool, observe fuel tank pressure while wiggling harness and ECM connector. If pressure changes more than -0.83 in. Hg while wiggling harness and ECM connector, check for poor terminal contact at ECM connector. If pressure does not change more than -0.83 in. Hg while wiggling harness and ECM connector, go to next step.
  6. Turn ignition off. On wagon models, move rear seat cushion. On all other models, remove rear seat cushion. Disconnect in-line connector located between rear harness and fuel tank harness. Turn ignition on. Measure voltage between ground and ECM-side of in-line connector terminal No. 5 (Baja, Legacy and Outback) or terminal No. 12 (Forester and Impreza). See Figure or Fig 1 . If voltage is more than 4.5 volts, go to next step. If voltage is not more than 4.5 volts, locate and repair an open circuit in wiring and check for poor terminal contact in connectors between ECM and 12-pin in-line connector. See ENGINE PERFORMANCE in appropriate SYSTEM WIRING DIAGRAMS article in ELECTRICAL.
    Fig 1: Identifying 16-Pin Connector
    G00233532Courtesy of SUBARU OF AMERICA, INC.
  7. Turn ignition off. Disconnect ECM 28-pin connector. Using an ohmmeter, measure resistance between ECM 28-pin connector, terminal No. 19 and ECM-side of in-line connector terminal No. 3 (Baja, Legacy and Outback) or terminal No. 14 (Forester and Impreza). See Figure and Figure or Fig 1 . If resistance is less than one ohm, go to next step. If resistance is not less than one ohm, locate and repair open circuit in wiring and check for poor terminal contact in connectors between ECM and in-line connector. See ENGINE PERFORMANCE in appropriate SYSTEM WIRING DIAGRAMS article in ELECTRICAL.
  8. Measure resistance between ground and ECM-side of in-line connector terminal No. 3 (Baja, Legacy and Outback) or terminal No. 14 (Forester and Impreza). See Figure or Fig 1 . If resistance is more than 500 k/ohms, go to next step. If resistance is not more than 500 k/ohms, locate and repair short to ground in wiring between ECM and in-line connector.
  9. Disconnect fuel tank pressure sensor connector. Measure resistance between fuel tank pressure sensor harness connector terminal No. 3 and fuel-pump side of in-line connector terminal No. 5 (Baja, Legacy and Outback) or terminal No. 12 (Forester and Impreza). See Figure, Figure and Fig 1 . If resistance is less than one ohm, go to next step. If resistance is not less than one ohm, locate and repair open circuit in fuel tank harness.
  10. Measure resistance between fuel tank pressure sensor harness connector, terminal No. 1 and fuel-pump side of in-line connector terminal No. 3 (Baja, Legacy and Outback) or terminal No. 2 and No. 14 (Forester and Impreza). If resistance is less than one ohm, go to next step. If resistance is not less than one ohm, locate and repair open circuit in fuel tank harness.
  11. Measure resistance between ground and fuel tank pressure sensor harness connector terminal No. 2 (Baja, Legacy and Outback) or terminal No. 1 (Forester and Impreza). If resistance is more than 500 k/ohms, go to next step. If resistance is not more than 500 k/ohms, locate and repair short to ground in fuel tank harness.
  12. Check for poor terminal contact at fuel tank pressure sensor connector. If terminal contact is okay, replace fuel tank pressure sensor. Clear memory and perform verification procedure. See CLEARING DIAGNOSTIC TROUBLE CODES  and VERIFICATION PROCEDURE  .
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