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Diagnosis & Repair

WARNING: This page is about a different car, the 2002 Lexus RX 300. However, it is still accessible from the selected car via links, so may be relevant.
  1. If other DTCs are displayed, diagnose and repair those DTCs first and retest system. If only DTC P0125 is displayed, go to next step.
  2. Check PCV piping. Repair as necessary. If PVC piping is okay, connect scan tool to Data Link Connector (DLC) No. 3. Start engine and raise engine speed to 2500 RPM for approximately 90 seconds. Using scan tool, monitor each A/F sensor output voltage. See AIR/FUEL SENSOR OUTPUT VOLTAGE SPECIFICATIONS  table. If voltage is as specified, go to next step. If voltage is not as specified, go to step  9.
    AIR/FUEL SENSOR OUTPUT VOLTAGE SPECIFICATIONS

    Application & Operating Condition Specification
    Using OBD-II Scan Tool
    Engine Idling, Engine Racing & Driving Vehicle (1) (2)
    Using Lexus Scan Tool
    Engine Idling, Engine Racing & Driving Vehicle (1) (3)
    (1) Drive vehicle at 25 MPH or more with engine speed of 1500 RPM or more while opening and closing the throttle.
    (2) Voltage should not remain at .56 volt or less, .66 volt, or .76 volt or more.
    (3) Voltage should not remain at 2.80 volts or less, 3.30 volts, or 3.80 volts or more.
  3. Check for open or short in wiring between ECM and suspect A/F sensor. See WIRING DIAGRAMS article. If problem exists, repair wiring as necessary. If problem does not exist, go to next step.
  4. Disconnect suspect A/F sensor harness connector. One A/F sensor is located in each exhaust manifold. Measure resistance between terminal B+ (Black/Red wire) and HT (Blue wire on bank No. 1 or Green wire on bank No. 2) at A/F sensor connector (component side). Measure resistance at both A/F sensors. Resistance should be .8-1.4 ohms at 68°F (20°C). If resistance is not as specified, replace appropriate A/F sensor. If resistance is as specified, go to next step.
  5. Ensure oil dipstick, oil filler cap, PCV system and all other air induction system components are intact and operating properly. If problem exists, repair as necessary. If problem does not exist, go to next step.
  6. Check fuel pressure. See FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article. If fuel pressure is not within specification, repair as necessary. If fuel pressure is within specification, go to next step.
  7. Check fuel injectors. See FUEL SYSTEM in SYSTEM & COMPONENT TESTING article. If problem exists, repair as necessary. If problem does not exist, go to next step.
  8. Check exhaust system for leaks. If problem exists, repair as necessary. If problem does not exist, replace defective A/F sensor.
  9. Using scan tool, clear DTCs. See CLEARING DIAGNOSTIC TROUBLE CODES . Perform test drive confirmation. See TEST DRIVE CONFIRMATION . Go to next step.
  10. Recheck for DTCs. If DTC P0125 is displayed again, replace ECM. If DTC P0125 is not displayed again, go to next step.
  11. Vehicle either ran out of fuel, or problem is intermittent. Check component and ECM connections.
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