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Diagnosis & Repair

WARNING: This page is about a different car, the 2002 Toyota Prius. However, it is still accessible from the selected car via links, so may be relevant.
  1. Using Toyota hand-held tester or scan tool, check for any other DTCs. If only DTC P0125 exists, leave hand-held tester or scan tool connected to DLC No. 3 and go to next step. If other DTCs exist, diagnose and repair those DTCs first and retest.
  2. Check for any loose or defective PCV valve piping connections. PCV valve is located on rear (firewall) side of valve cover at flywheel end of valve cover. If PCV valve piping is okay, go to next step. If PCV valve piping is defective, repair or replace components as necessary.
  3. With Toyota hand-held tester or scan tool still connected to DLC No. 3, start engine and warm engine to normal operating temperature. Using hand-held tester or scan tool, monitor heated oxygen sensor No. 1 output voltage while suddenly accelerating engine to 4000 RPM for 3 consecutive times. Heated oxygen sensor No. 1 should indicate a rich condition by displaying output voltage of .45 volt or more at least once. If rich condition is not indicated by heated oxygen sensor No. 1 at least once, go to next step. If a rich condition is indicated by heated oxygen sensor No. 1 at least once, go to step  10.
  4. Check for open and short in wiring harness and electrical connectors between heated oxygen sensor No. 1 and ECM. See WIRING DIAGRAMS article. Heated oxygen sensor No. 1 is located on exhaust pipe in front of catalytic converter. ECM is located behind glove box. See Figure. If wiring harness and electrical connectors are okay, go to next step. If wiring harness or electrical connector is defective, repair as necessary.
  5. Using hand-held tester or scan tool, check if a misfire has occurred by monitoring DTCs and DATA LIST. If misfire has not occurred, go to next step. If a misfire has occurred, perform DTC P0300: RANDOM/MULTIPLE CYLINDER MISFIRE DETECTED  or DTCS P0301-P0304: CYLINDERS NO. 1-4 MISFIRE DETECTED .
  6. Ensure engine oil dipstick, oil filler cap, PCV system and all other air induction system components are intact and operating properly. If problem does not exist, go to next step. If problem exists, repair as necessary.
  7. Check fuel pressure. See PRIUS under FUEL PRESSURE under FUEL SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4-CYLINDER article. Fuel pressure should be 44-50 psi (3.1-3.5 kg/cm2 ) with engine idling and should remain at least 21 psi (1.5 kg/cm2 ) for a minimum of 5 minutes after engine is shut off. If fuel pressure is within specification, go to next step. If fuel pressure is not within specification, repair fuel system as necessary.
  8. Check operation of fuel injectors. See FUEL CONTROL under FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 4-CYLINDER article. Fuel injector volume should be 36-46 cc (2.1-2.8 cu. in.) within 15 seconds and difference in volume between each fuel injector should be less than 10 cc (.61 cu. in.). Fuel injectors should not leak more than one drop every 12 minutes. If fuel injectors are okay, go to next step. If fuel injectors are defective, replace fuel injectors as necessary.
  9. Check exhaust system for leaks. If no exhaust leak exists, replace heated oxygen sensor No. 1. See HEATED OXYGEN SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - 4-CYLINDER article. If exhaust leak exists, repair exhaust system as necessary.
  10. Using Toyota hand-held tester or scan tool, clear DTCs from ECM. Perform test drive confirmation and then go to next step. See TEST DRIVE CONFIRMATION .
  11. Recheck for DTCs. If DTC P0125 does not exist, go to next step. If DTC P0125 exists, replace ECM. ECM is located behind glove box. ECM is located behind glove box. See Figure.
  12. Verify if vehicle ran out of fuel. If vehicle ran out of fuel, DTC P0125 was caused by the vehicle running out of fuel. If vehicle did not run out of fuel, problem is intermittent. Check for defective connections or intermittent problem in wiring. ECM is located behind glove box. See Figure.
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