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DTC P0420 - CATALYTIC CONVERTER EFFICIENCY BELOW THRESHOLD Mitsubishi Galant VIII

Testing & Diagnostics ~224 words
  1. Check exhaust manifold for cracks. If exhaust manifold is okay, go to next step.
  2. Using scan tool in MUT II mode, read SERVICE DATA No. 59 (rear heated oxygen sensor). Drive vehicle with transaxle in Second gear (M/T) or Low Range (A/T), and with wide open throttle. If voltage is 600-1000 millivolts, go to next step. If voltage is not as specified, check rear Heated Oxygen Sensor (HO2S) circuit. See DTC P0136:REAR HEATED OXYGEN SENSOR (HO2S) CIRCUIT MALFUNCTION
  3. Using scan tool in MUT II mode, read SERVICE DATA No. 11 (front heated oxygen sensor). Monitor rear HO2S output voltage while repeatedly racing engine. If voltage is 600-1000 millivolts, go to next step. If voltage is not as specified, check front HO2S circuit. See DTC P0130: FRONT HEATED OXYGEN SENSOR (HO2S) CIRCUIT MALFUNCTION
  4. Using scan tool, perform SENSOR READ TEST No. 11 (front heated oxygen sensor volts). Monitor front HO2S output voltage with engine at idle. Scan tool reading should alternate between 0-400 millivolts and 600-1000 millivolts. If voltage is as specified, go to next step. If voltage is not as specified, replace front HO2S.
  5. Replace rear HO2S. Clear DTCs. Road test vehicle to verify repairs. If DTC 0420 resets, replace catalytic convertor. Clear DTCs. Road test vehicle to verify repairs. If DTC 0420 resets, replace PCM.

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