So I changed the spark plugs/wires and the cyl 4 fuel injector and the light and code went away for a week, popped back up ( even though there is no actual misfire now ) and it keeps coming on, again there is definitely not a misfire, why is the code still popping up? I know I need to replace the upstream o2 sensor, could that be a problem with changing the fuel injector since the upstream o2 is fuel related?