2000 Other Acura Models chk engine light on

I got Acura Integra 2000 having 118000 Miles on it. Two weeks back the chk engine light turned on with OBD code PO420, the mechanic changed the catalytic convetor assuming cat convertor performingunder efficiency and he advised me to drive 100 miles, but the chk engine again turned on after 50 miles. Next time he says there is crack and o2 sensor not functioning, which he welded and also replaced oxygen sensor. It worke fine for 300 miles but today again chk engine light on, I am really frusterated, pl suggest wt can be the reason.