Question about 1997 Ford Taurus

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I've got a CEL P0420 code on a 96 Civic. I've used a scanner tool, and it appears to be a Catalyst issue based on the O2 sensor readings. Both follow about the same frequency. However, I am seeing a Closed Loop status even at start up. I would expect it to have an Open Loop when the engine is cold. What can I check to verify the engine temperature is being assessed properly? I want to make sure another problem isn't present before replacing the Catalyst. Thanks.

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This code can mean any of the following and not a necessarily bad catalytic converter

A code P0420 may mean that one or more of the following has happened:

  • Leaded fuel was used where unleaded was called for
  • An oxygen sensor is not reading (functioning) properly
  • The engine coolant temperature sensor is not working properly
  • Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
  • Retarded spark timing
  • The oxygen sensors in front and behind the converter are reporting too similar of readings
Check the following
  • Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
  • Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
  • Inspect the downstream heated oxygen sensor (HO2), replace if necessary
  • Replace the catalytic converter

Posted on Nov 13, 2009

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