Ok I bought my 03 trailblazer in 09 first major repair was a new transmission under warranty I complained of the rattling muffler from the begging and a smell of sulfur. I have replace 3 coils 1 in 2010 and 2 more March of 2015 all different numbers now it's running rough again. They never changed the plugs. The check engine light is flashing it will run great for several days then start running bad again for a day or so and stop. Took it to auto zone the said I have a misfire in #3 now. Is something causing the coils to misfire or is this normal should I try plugs since it's been a while? I already had a fuel pump replaced in march right before the #4&#1 coil in march as well .
And what about 02 sensor?