Toyota Repair: P0420 diagnostic code, excessive oil consumption, oxygen sensors

Question
I have a 1996 Toyota Camry with a 3.0 V6. The check engine light is on again. The catalytic converter was changed twice. The previous owner replaced it about a year ago. The car has 171,820 well maintained miles. I just had the converter changed again about two months ago.  The garage said that the one the previous owner installed was a cheaper inferior one.  They installed "a better quality" after-market converter after explaining the difference. Two of the oxygen sensors have been replaced.
 The engine, since a few weeks ago started using oil. The car could sit from just twenty five minutes to a cold first start of the day. Upon ignition, a puff of blue smoke emits from the tail pipe.  The smoke issue is intermittent, Could this cause the converter or sensors to fail? Or is the code something else?  Thank You

Answer
Hello, yes the excessive oil consumption can cause a converter to overheat and fail over time, the blue smoke on startup is common on these engines, usually caused by leaking valve stem seals, the seals can be replaced without removing the cylinder heads but at this mileage I would recommend removing the heads and doing a complete valve job, some V6 engines also have a tendency to develop excessive sludge or oil gelling, it can be inspected for this by removing the front accessible valve cover.