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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. Ensure engine oil dipstick, oil filler cap, Positive Crankcase Ventilation (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.
  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. 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.
  4. Check Mass Airflow (MAF) meter and Engine Coolant Temperature (ECT) sensor. MAF meter may also be referred to as airflow meter. See AIRFLOW METER and ENGINE COOLANT TEMPERATURE SENSOR under ENGINE SENSORS & SWITCHES in SYSTEM & COMPONENT TESTING - 4-CYLINDER article. If MAF meter and ECT sensor are okay, go to next step. If MAF meter or ECT sensor is defective, replace components as necessary. See AIRFLOW METER or ENGINE COOLANT TEMPERATURE SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - 4-CYLINDER article.
  5. Check ignition system by performing spark test. See SPARK TEST under IGNITION SYSTEMS in BASIC DIAGNOSTIC PROCEDURES - 4-CYLINDER article. If ignition system operates properly and spark exists, go to next step. If ignition system does not operate properly and spark does not exist, repair ignition system as necessary.
  6. 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.
  7. Check exhaust system for leaks. If no exhaust leak exists, go to next step. If exhaust leak exists, repair exhaust system as necessary.
  8. Connect Toyota hand-held tester or scan tool to Data Link Connector (DLC) No. 3 at driver's side of instrument panel. See Figure. Start engine and increase engine speed to 2500 RPM for approximately 90 seconds and then allow engine to idle. Using hand-held tester or scan tool, monitor heated oxygen sensor No. 1 output voltage with engine idling. Output voltage should alternate repeatedly from less than .40 volt to more than .55 volt. If output voltage is not as specified, go to next step. If output voltage is as specified, go to step  10.
  9. 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, replace heated oxygen sensor No. 1. See HEATED OXYGEN SENSOR under ENGINE SENSORS & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION - 4-CYLINDER article. If wiring harness or electrical connector is defective, repair 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 P0171 or P0172 does not exist, go to next step. If DTC P0171 and/or P0172 exists, replace ECM. ECM is located behind glove box. See Figure.
  12. Verify if vehicle ran out of fuel. If vehicle ran out of fuel, DTC P0171 and/or P0172 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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