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Home >> Toyota >> 1997 >> Previa DX, AWD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System Tests W/Codes >> Circuit Tests >> DTC P0300: Random Misfire Detected >> Diagnosis & Repair
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Diagnosis & Repair

  1. Inspect spark plug and check for spark at misfiring cylinder. See BASIC TESTING article. Repair as necessary. If spark plug is okay and spark is present, go to next step.
  2. Access ECM under driver's seat. Turn ignition on. Using DVOM, backprobe ECM connector. Measure voltage between ground and terminals No. 11 (Black/Blue wire) and No. 12 (Black wire) at ECM E8 connector. See Figure. If voltage is 9-14 volts for each circuit, go to step 4). If voltage is not 9-14 volts, go to next step.
  3. Disconnect injector of misfiring cylinder. Using ohmmeter, measure resistance between injector terminals. If resistance is 13.4-14.2 ohms, check for open or short circuit in wiring harness between ECM and injector. See appropriate wiring diagram in WIRING DIAGRAMS article. Repair wiring as necessary. If resistance is not 13.4-14.2 ohms, replace injector.
  4. Check fuel pressure. See BASIC TESTING article. Repair fuel pressure as necessary. If fuel pressure is okay, check fuel injector performance. See SYSTEM/COMPONENT TESTS article. Replace fuel injectors as necessary.
  5. If all injectors are okay, check EGR system. See SYSTEM/COMPONENT TESTS article. Repair EGR system as necessary. If EGR system is okay, check MAF sensor and ECT sensor. See SYSTEM/COMPONENT TESTS article. Replace as necessary. If both components are okay, check engine mechanical condition (compression, valve clearance, valve timing, etc.). See BASIC TESTING article.
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