303 code problem

Question

Originally had codes 135,141,301,303,305,306. Replaced both bank 1 O2 sensors. All plugs and wires, being a waste spark system with 3 coils firing all 6 cyls. After all of this code303 still shows up. Im thinking Cyl #4 coil which also fires #3 cyl may be bad. If an injector was bad I would think it would throw other codes maybe 171 or 172 with the 303. I'm probably going to replace cyl #4 coil but I would appreciate some input before I proceed