my car, 2006 cobalt ls 2.2 completely stock, today out of no where the cel came on, car has been running perfectly fine and still does, took it to part store and scanned the code p0326 (knock sensor). iv been using regular 87 gas in it, and i have been doing a tune up so far iv done fuel filter,oil change and pulled the TB and cleaned it off. this all happened last week. any advise?

 

Plugs

 

i was thinking that, its got 102k and there the stock plugs so i thought maby they were bad. its only averaging 28mpg so i was planning on them next.