My Honda Civic August 2012 has had an error code P0420 and Honda replaced an alleged faulty O2 sensor downstream. One replaced they then got the same error code (P0420) stating Cat below threshold. Does the cat need replacing or can it be cleaned. The car does use a lot of oil. Any thoughts or advice?

My first thoughts are you have a slight exhaust leak before the cat, or even a crack in the manifold.

The only true way to determine this is to either place a rag firmly over the exhaust and listen for escaping air....there's your leak!

Alternatively, if you have a friendly garage, ask them to measure the oxygen levels in the exhaust gases and if much more than 0% to 1%  you have air in the gas which means its drawing it from somewhere and not burning it (post combustion)

Let us know what you find