new guy need help with carrier unit

hi,i am new here but i really need some help.
i have a problem with carrier ultra xtc unit.
the problem is when the unit is running start/stop mode and reach setpoint, it is cutting of,but the actual box temp. aprox.10-12 deg. higher.
exm. set temp 0 deg ,unit cut of 0 deg.,but actually the box temp 12 deg
please help
tx