Ive changed out sensors only to have the new ones be bad as well... Its a pain

 

Double check your coils! Just went through hell and a fortune to fix the misfire on mine. Started by replacing new plugs and coils, still flashing cel, so I replaced injectors problem persisted. Thought it coud be kcked rocker arm or valve spring, dissambled head not the issue. I ended up with o2 sensor codes so replaced them all. Then was thinking could be the head gasket and gave up for a while. My daughter ran into a dude who said his was the heater control sensor in the intake so I replaced that, I noticed a big improvement on how it ran after I did that but still had the code. Found an article online about a tsb for wrong spark plug gaps so I decided to go back to them. Bought new plugs gapped at 35 acdelco 41-602 originally used ngk iridiums. Would recommend the acdelcos oem part. Misfire 0304 flashing. Ready to lose my mind found a used motor and was ready to do a swap instead I took the coil out of that engine and swapped with number 4, bang code gone runs awesome! Over the process I have learned a ton about these motors theyre picky as hell.