Make sure you use high temperature nickel antiseize on the ground barrel of the new plug after thoroughly cleaning the bore.

 

I am getting a CEL code p0430 which looks to be the catalytic converter or O2 sensor. I had both converters and sensors replace a little over a year ago at a shop near by. Could it be something else?

Also the gas mileage got worse after the tune up, maybe it has something to do with the catalytic converter? Still less than 12 mpg. Thanks.