1998 Honda Accord check Engine light

Hi, my check engine light has been on for awhile, so naturally when I went for inspection I failed the report said for emmisions and the code was for my exhaust system, took it the mechanic and he looked it up said the code was for the catalytic converter they also put in new o2 sensors. Mechanic told me to wait about 48 hrs b4 having it reinspected because the new parts had to cycle through. I left the shop check engine light was off later on in the evening after driving about 100 miles for the day my light came back on. Why is this happening could there be something else wrong?