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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 and transmission 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 50-62 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 indicated, check three way catalytic converter (TWC). Replace if necessary. If temporary DTC P0420 is not indicated, check for readiness completion. If readiness code is complete, it was an intermittent failure, system is okay at this time. If readiness code is incomplete, repeat step one.
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