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DTC P0420: Catalyst System - Efficiency Below Threshold

NOTE: Trouble shoot DTC P0137, P0138 or P0141, if set, before proceeding.
  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.
  2. Connect scan tool to vehicle. Test drive vehicle at 40-55 MPH for at least 2 minutes. Decelerate with throttle completely closed for at least 3 seconds. Reduce speed to 35 MPH and try to hold until scan tool indicates FAIL or PASS as TWC test result. If test result fails, check TWC and repair as necessary. If test result does not fail, problem is intermittent. System is okay at this time.
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