'02 mustang gt po304 code keeps coming on

Question

I have changed spark plug, ignition coil, checked manifold gasket, checked compression and the check engine lite still comes on after 2 days! Also the battery icon has been coming on and off since I put a rebuilt motor and transmission in. When clutch changed the PO304 HAS BEEN COMING ON. There is nothing wrong with the spark plug and may have lost 20% of its power since the motor was put in. I'm ABOUT TO PUT A PIECE OF TAPE OVER THE CHECK ENGINE LITE. I may have alittle misfire but not much . If the O2 sensor is bad wouldn't that show up in the codes?

Answer 1

The other possible issue could be a clogged fuel injector or a wiring issue, I would use a good fuel system cleaner like 3M or BG 44k and maybe do an intake flush. 

you can also check injector resistance on cylinder 4 and compare it to other cylinders (also the voltage)