I have a dtc p0303. The spark plugs and wires has been changed what next? on 1998 Chevrolet Express 1500

service engine light still remains on

After repair the computer will have to be reset to turn off SEL. If you have done this and still have the same code may have an injector problem. Or as answer 1 said compression low.
1 more answer
check the compression on that cylinder and let me know what u find.