I am just posting this to share my diagnostic nightmare that I went through with my blazer at 125,000 miles. First i threw the dtc p0300 randomly and thought nothing of it cleared it and it didnt come back for weeks. I was driving to GA from IL for vacation and it started flashing in Indiana. Threw plugs wires cap and rotor at it. It was fine for 200 more miles then SES started flashing again. Limped it to GA mile by mile.Got ahold of a Snap On Modis tool that my friends shop had and found out i now had p0303 p0304 p0300 etc. So i checked all my wiring from my tune up,all good. Watched the misfire data and found cyl 3 and 4 were missing every turn. Went into active tests and ran the fuel injector pressure releif test. Found 3 and 4 had extremely high resistance and would not open at all. Thusly I got ahold of a wonderful spider unit replaced the assembly in about 3 hours cleared the codes and now she runs just fine!! Hopefully this helps someone!