oxygen sensors (cat conv)

I drive a 95 v8 cougar and for the past 6 months there has been a rattling in my exaust system when I accellerate. My friend had the same problem in his 97 f150 and found out that pieces of the cats had broken off and when he accellerated it moved around in the exaust. Recently my check engine light came on and autozone told me it was my oxygen sensor. Is it necessary for me to change my cats? is something going to go wrong if I leave this alone?

the reason your cats have gone bad is because your 02 sensors are bad. Expect to see decreased performance as well as lower gas mileage.