My daughter's 2005 Envoy (145k miles) had the check engine light come on a couple of days ago (very cold out......around 5 degrees). It is idling kind of rough and the check engine starts flashing when I get it above 30 MPH. I took it to the auto part store and they ran a diagnostic and it is showing code P0306 (Cylinder 6 Misfire Detected). Is my first step to replace the spark plugs (it's been over 50k since they have been replaced)? Thanks!