The o2 sensor have 12 volt at key on and the signal wire was 5 volts at the computer and it still run very rich. My mustang run without the o2 sensor plugged in

Please Help

 

That might be your problem.......if you're checking the O2 signal wire, it shouldn't have 5vdc, it's the O2 that generates the voltage fluctuation between 0 and 1 volt, 0.46vdc is the lean/rich threshold.
From the little information provided, you may have a shorted O2 signal wire to VREF somewhere in the wiring or a bad ECM. Hope this helps. GL