Explorer Codes

1996 Ford Explorer Sport, 2 Door. 6 Cylinder, 4 liter, 235K miles. CEL is on. Code reader yields the following : P0420 Catalyst efficiency below threshold Bank 1

The four main possibilities are (in no particular order):

  1. The bank 1 catalytic converter is failing.
  2. The bank 1 catalytic monitor O2 sensor is failing.
  3. The bank 1 engine O2 sensor is failing (probably should replace both if you go this way.)
  4. Exhaust leak.

Thanks - will probably replace those 2 O2 sensors.