T957_O or A957_O: Low Refrigerant Pressure
On the outdoor unit control, this will be indicated by a number 21 or L21 on the seven segment LED display depending on whether the unit has shut down and locked out.
This alarm means that the low pressure switch has opened and the unit has shut down. The system is allowed 3 low pressure switch trips on a single cycle before locking out for one hour. Check system charge and for restrictions or an improperly operating metering device. If metering device is suspected to be the root cause and if this system is matched with an EEV coil and this fault continues, an alternate solution may be to verify the indoor EEV control board part number ends in -02 (furnace coils only) or install a matching TEV coil.
