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Home >> Oldsmobile >> 1997 >> Regency >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control Systems Tests W/Codes >> Diagnostic Trouble Codes >> DTC P0420 - TWC System Low Efficiency >> Diagnostic Procedures
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Diagnostic Procedures

  1. Perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK. See the BASIC TESTING - 3.8L article. Read and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. Go to next step.
  2. Diagnose any other DTCs present before proceeding. If no other DTCs are present, go to next step.
  3. Ensure TWC is an original equipment part. Check TWC for dents or discoloration caused by excessive heat. Ensure no internal damaged catalyst rattle exists. If a problem is found, repair as necessary. After repairs, go to step 6). If no problems are found, go to next step.
  4. Check exhaust system for leaks, damage, loose or missing hardware. If a problem is found, repair as necessary. After repairs, go to step 7). If no problems are found, go to next step.
  5. Ensure HO2S 2 is securely installed and that harness is not damaged or contacting exhaust. If a problem is found, repair as necessary. After repairs, go to step 7). If no problems are found, go to next step.
  6. Replace TWC. Check for possible engine misfire DTC or engine mechanical problem, and go to next step.
  7. Read and record FAIL RECORDS data for DTC P0401. Clear DTCs. Start engine and allow it to reach 167°F (75°C). Run engine while monitoring scan tool IAC RPM control function to maintain MAF at 12-15 grams per second for at least 2 minutes. Using scan tool, monitor TWC MONITOR TEST COUNTER while operating vehicle within conditions required to set DTC. Continue operating vehicle until TWC MONITOR TEST COUNTER reaches 49 and returns to zero 2 times. Read SPECIFIC DTC. If scan tool displays DTC P0420 TEST RAN AND PASSED, repair is complete. If scan tool does not display DTC P0420 TEST RAN AND PASSED, return to step 2).
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