I have a girlfriend who has a trailblazer that has over 200,000 miles on it and has had an SES light on for sometime now. Occasionally the light will be blinking. I have had the local auto parts store scan the computer for her and it came back cylinder three misfire. I have since switched the coil packs out from cylinder three to cylinder one and have also switched the spark plugs from cylinder three to cylinder two. I could barely notice the knocking anymore and the car seems to idle a little bit better. I then proceeded to go ahead and change all of the plugs out with new AC Delco iridium plugs. After changing all the plugs, I started it up again and it has a very noticeable knock again and runs and idols horribly. I cleared the codes and waited for the SES light to come back on and went back to the auto parts store to have it checked. Still has a misfire on cylinder three. Bought some gum out injector cleaner and put some 93 octane gasoline in the car with it and I am currently driving it around waiting to see if it clears up. Anyone else have any thoughts on what the problem could be? I forgot to mention that she has to add oil from time to time and is currently 10k since her last oil change.