Ive got a 99 w/ 4.2 the driver side header has been cracked badly for awhile but the truck still ran fine. I finally got around to changing it out as well as the 02 sensor. One stud was broken off and had to be drilled/tapped to 10mm. Afterwards the truck ran very rough, made a popping noise, the driver side cat turned red, and would barley go 25. I changed both cats and still had no luck. Pulled the head off today to see if drilling it affected it any and it seems fine. Anyone have an idea as to what could be going on? Changed the plugs as well

 

did it throw a code?

 

It did throw a code but I disconnected the negative battery cable when changing the cats to try n let it clear hoping the cats were the problem. After doing so I didnt drive it far enough (about 1-1.5 miles) to make it throw a code again as the new cat was already red hot and I didnt want to burn it up. Is it possible the code could still be stored since techically it wasnt cleared, just the negative cable disconnected.