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Home >> Toyota >> 1997 >> Previa DX, RWD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System Tests W/Codes >> Circuit Tests >> DTC P0401: Insufficient Exhaust Gas RECIRCULATION (EGR) Flow Detected >> Diagnosis & Repair (Using Toyota Scan Tool)
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Diagnosis & Repair (Using Toyota Scan Tool)

  1. Connect scan tool. See Figure. Turn ignition on. Read EGR gas temperature. If temperature is 50°F (10°C), go to step 5). If temperature is not 50°F (10°C), go to next step.
    NOTE: If EGR gas temperature is 37.6°F (3.1°C), an open exists in EGR temperature sensor circuit.
  2. Disconnect EGR temperature sensor connector. Connect jumper wire between wiring harness connector terminals. If EGR temperature displayed is not about 316°F (158°C), go to next step. If temperature displayed is about 316°F (158°C), check sensor connectors and terminals. Repair as necessary. If connectors are okay, replace sensor and retest.
  3. Remove jumper wire. Turn ignition off. Access ECM under driver's seat. Connect jumper wire between terminals No. 12 (Brown/White wire) and No. 9 (Brown/Black wire) at ECM E6 connector. See Figure. Turn ignition on.
  4. If EGR temperature displayed is about 316°F (158°C), locate and repair open circuit in wiring harness between ECM and EGR temperature sensor. If temperature displayed is not about 316°F (158°C), replace ECM and retest.
  5. Check vacuum hoses. See appropriate illustration in VACUUM DIAGRAMS article. Replace vacuum hoses as necessary. If all hoses are okay, check VSV. Using scan tool, select ACTIVE TEST mode. Check operation of VSV when activated by scan tool. VSV is located near throttle body.
  6. Start and run engine. With EGR system off, air from port "E" should flow from filter on end of valve. See Fig 1. With EGR system on, air from port "E" should flow from port "F". If VSV operates as specified, go to step 9). If VSV does not operate as specified, go to next step.
  7. Remove EGR VSV. Using ohmmeter, measure resistance between VSV terminals. Resistance should be 37-44 ohms at 68°F (20°C). Check for continuity between each VSV terminal and body of valve. If continuity exists, replace VSV. If continuity does not exist, go to next step.
  8. Check VSV operation. Apply battery voltage to VSV terminals. Apply air pressure to VSV port "E". Air from port "E" should flow from port "F". See Fig 1. Remove battery voltage from valve terminals. Air from port "E" should flow from filter on end of valve. Replace VSV as necessary. If VSV operates as specified, check for an open in wiring harness between EGR VSV and ECM. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair wiring as necessary.
  9. Check EGR vacuum modulator. See SYSTEM/COMPONENT TESTS article. Replace as necessary. If modulator is okay, check EGR valve. See SYSTEM/COMPONENT TESTS article. Replace as necessary. If components are okay, go to next step.
  10. Ensure all components are connected. Start and run engine. Using scan tool, select ACTIVE TEST mode (EGR system ON). Run engine at 3500 RPM for 3 minutes. Monitor EGR gas temperature. If temperature is 284°F (140°C) or more, replace ECM and retest. If temperature is less than 284°F (140°C), replace sensor.
Fig 1: Identifying EGR Vacuum Switching Valve (VSV)
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