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OBD-II Trouble Shooting Got a code reader? Find out what your GM car's Check Engine light is trying to tell you (and then turn Check Engine light off).

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Old 09-24-2009, 07:15 PM   #1  
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Default P0420



Three-Way Catalyst (TWC) system is used to control emission.
PCM uses signal from Heated Oxygen Sensors (HO2S) to read efficiency
of TWC. PCM will set DTC P0420 if TWC oxygen storage capacity is less
than a predetermined threshold.

You will likely not notice any drivability problems.

  • An oxygen sensor is not working properly
  • The engine coolant temperature sensor is not working properly
  • Damaged or leaking exhaust manifold system
  • Retarded spark timing
Possible Solutions
  • Check for leaks in the exhaust system, manifold, pipes, catalytic converter. Repair as required.
  • Use a scanner to test the O2 sensor for proper wave form.
  • Inspect the oxygen sensor, replace if necessary
  • Replace the catalytic converter
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obd2, obdii, p0420, p420

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