CL4u2NV

My check engine light is still on, and I tried geting a code and it gave me a code for an O2 sensor. So what I did i went a baought a O2 sensor. I replaced and reseted the computer and ran it for a couple of miles. After I ran the car the light came on again. I was lucky the guy who was suppose to smog my car had an OBDII Scan tester. He then told me that my Catalytic is bad and that 2 other sensors are bad. When he looked up a code that the scan device gave him, he said something about the cat being below threshold, but i forget, and I dont know what he means? Anyone have a clue what can be a problem , or what i need to replace in order for me to get my car smogged. I dont want to keep guessing and buying parts i dont need. My car is a 1998 with about 160,000 miles. Its a grip of miles but still runs good, just want to pass smog before the deadline for the DMV. Thanks For everyones and anyones help. Please respond back to me asap!