Good morning, Yesterday when driving down the interstate the check engine light started flashing and when giving throttle to accelerate to pass the vehicle started shaking badly. The truck has 126k miles, I'm the second owner. Limped it to home, Codes showed P0303, swapped coil packs and the code stayed 303, put it all new plugs with Bosch Iridium and still 303 code. Permanent 303 code with all new plugs and the misfire is still obviously there, chassis is shaking from misfire. Power seems to be weak until 2k-4k rpm and will run "smooth" not under strain. Looking for the next step in either diagnostics or repair thanks.

Crazedshifter

 

Since your description indicates it's contained to one cylinder, and coilpak has been eliminated, the injector is the next logical spot to check/verify. Look all of the cyl 3 wiring over for obvious signs of damage (such as animals chewing on it) or for loose/corroded terminal connectors.