P0420 Catalyst System Bellow Threshold Bank 1?

ok, first off be very careful on "cheap" parts as sometimes they are crap. On O2 sensors, buy name brand with the plug attached. Sometimes the instructions are wrong which can lead to killing the ECU. BTDT. I would try the cat additive and also consider pulling the MAF and cleaning it with MAF spray cleaner. It is easy, may require a torx bit but worth having as the MAF cleaning should be an annual thing. Also, if it the O2 sensor does turn out to need replacement proceed carefully. They can be rather tough to break loose and have been known to strip out the threads in the pipe.
On oil consumption, with your mileage if the car was not well maintained it may just be a worn out motor. Some will try an oil ring cleaning procedure, which is cheap but time consuming. You can find lots of BTDT on this forum.
Hope this helps more than some of the other posts!

 

I did not say it has to be welded. Reread.

None of the cat or oil burning additives work. However a clogged cat can cause engine failure and fires.

 

Catalytic converter on ebay is ~$100 with 5 year warranty. Rock auto have it for ~$250.

Does any one tried any of these after market brands?