Got a code for the catalytic converter
 

I think it said p0420 or something like, but the guy said it had to do with the CC. I just had an exhaust system put on about 1.5 months ago past the the CC, so can it still effect it? If the problem is really the CC, what can I do about it? Is it expensive? Please help. Everytime I fix onething, something else goes wrong. Damn it!!! Thanks for your help

CC bank 1 efficiency threshold has been reached.
How old is the truck?
What have you been running for gas?
Does it use any oil?
Losing any coolant?
The cats can be pretty pricey.
A test of sensor switching is in order so you don't get fooled by the sensors being lazy or contaminated.
Something you could do is swap the sensors side for side, clear the code and see what returns to see it the side changes or stays the same then go from there.

I have a 1999 5.4 F150. I run regular unleaded and maybe 2 times within the past few months used the 89 octane.
What do you mean does it use any oil?
I had the coolant changed a month ago
Is switching the sensors easy and easy to locate?
Thanks

cc are covered 8 years-80 thousand miles.How many miles due you have?I always reprogram pcm and retest.If cc still fail with p0420.replace it.they come separate.replacing o2 sensors will not fix it.