Misfire in cylinder 4

2001 Jeep Grand Cherokee Laredo. 4.0 L
No problems for a year. Then misfire in cylinder 4 -P0304 and Injector 4 Control Circuit -P0204. Changed spark plugs -ran okay for a few weeks-still engine light on, changed ignition coil -found out spark plugs not tightened enough. Fixed that, ran well for less than a week. Check engine light back on. P0304. I don't have equipment to test the injector, so I assume the most economical option would be to go ahead and replace it. After that maybe a computer problem? I don't really understand car computers, but when the check engine light is on, it runs rough, so logically it cant be a sensor problem?

You have a while and more testing to do before you blame the computer. Swap the injector with a different cylinder and see if your miss moves.

If the swap doesn't work purchase a noid light at the parts store for your vehicle and check for pulse to injector.
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