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DTC P0420 - Catalyst Efficiency Below Threshold

  1. Install scan tool. Start engine and check if DTC P0420 is present. If DTC is present, go to next step. If DTC is not present, problem is intermittent. See INTERMITTENTS in TESTS W/O CODES article.
  2. Check for exhaust leak before and after the catalytic converter. Repair as necessary. If leaks are not found, check rear HO2S. For diagnosis, see related DTC or ENGINE SENSORS & SWITCHES in SYSTEM/COMPONENT TESTS article. Replace as necessary. If rear HO2S is okay, go to next step.
  3. Replace catalytic converter. Clear DTC. See CLEARING CODES under Self Diagnostic System section. Retest system. If DTC resets, replace ECM.
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