For the first time on my truck with 123000 on it my service engine light came on. Got the codes PO304 and PO307. Said fault cylinder on cylinders E and C. Any ideas what this means? Thanks

 

It means service engine lights are pieces of crap who's main purpose is to irritate and scare people. No one needs them if they do the suggested service at each mileage, watch their gauges, and check the engine on a regular basis for problems.
I HATE THE CHECK ENGINE LIGHT.
Had one on an Oldsmobile lit up, they never could find out what the problem was, so I just disconnected it and drove the car another 60,000 miles with no problems.

But to answer your question, it is probably the spark plugs, or it could be the COPs. I would change the plugs first, reset, and see what happens.