Hoping someone could shed some light on this fault code on a lennox unit.

Unit is gas heat/ cooling system. Every so often unit faults on code 13. (inlet or room t. Temp to low.) Controller is a dc 50 comfort. Condensing unit is BGK070HD Could anyone shed some light what could cause this code. Any one who works with lennox may find this a simple one? Just been out to the unit its pitch black where the unit is so will probably go back tommorow if i have time. Thanks guys