First check whether the thermostat allows you to access the change mode screen. If not, locate the installation's master thermostat ( Which is my installation's master thermostat? ).

If you are indeed accessing from the master thermostat and the problem is that it selects a mode other than STOP and the system automatically reverts back to STOP, the problem could be in the main control board. Check whether there is a bridge at connector J14 (ALAR) in the main control board and that it is actually being short-circuited (check with a multimeter). If it is not, short circuit the two poles of connector J14.