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DTC P0420: Catalyst System Efficiency Below Threshold: MDX

  1. Perform PCM reset procedure. See PCM RESET PROCEDURE/CLEARING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. Start engine and run at 3000 RPM, with no load (in park or neutral) until radiator fan comes on. Drive about 10 minutes on highway without stopping. Speed can vary. With transmission in "D5" position, drive at steady speed between 50 and 62 MPH for 30 seconds. Repeat three times, closing throttle completely for 1-2 seconds between each repetition. If engine is stopped during this part of procedure, start over with road test.
  2. Check for temporary DTCs with scan tool. If DTC P0420 is set, check Three Way Catalytic Converter (TWC). Replace if necessary. If temporary DTC P0420 is not set, check for readiness code completion. If readiness code is complete, problem is intermittent, system is okay at this time. If readiness code is incomplete, repeat procedure.
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