So about three weeks ago I replaced plugs and wires on my 01 Tahoe. Then just the other night the SES light came on, so I went to Autozone and pulled the code; it was throwing:
P0300 (random/multiple cylinder misfire)
P0420 (Catalst effeciency below threshold)
and P0420 Pd Bank 1.

Is it throwing these codes because the problem wasn't fixed? Or do I just need to reset the computer?
I'm lost and any help would be supremely appreciated.
Thanks guys

 

guarantee your catlytic converter is clogged causing the misfire.
i just dealt with this same exact codes and problem at work on a 00 gmc yukon.
you know GM gives you a 8 year 80000 mile warranty on emissions equipment, so if you're under that, go to the dealer and it'll be free(or atleast pretty cheap).
you could either back pressure test your exhaust through the 02 sensor hole or just drop off the y pipe that has both cats and see that it runs fine with out them.

 

That makes a lot of sense actually. Because the Cat is clogged it is clogging the cylinder with exhaust? i.e. too much back pressure?
****, I am at 108,000 miles.
How much is a new Cat gonna cost me? Ballpark figure.