What does it mean to service engine soon?

I have a 2006 Mustang GT with automatic trans that the service engine soon light came on. The code indicated O2 sensor with possible causes such as MAF sensor, fuel filter, fuel pump and vacuum leak. I replaced all 4 O2 sensors, but the same code was still being triggered after disconnecting battery cable for 1 hour and driving the vehicle over 50 miles. I then changed the fuel filter, same thing, then changed the MAF, same thing and the fuel pump had been replaced by FORD last year which did not trigger a code to begin with. The pump charges strong. I also checked for vacuum leaks and could not detect any. What else could be causing the same code activation that I might be overlooking?

Thanks,

Lee