Hi Bejarano, as you comment, one of the simplest reasons why that thick return pipe is frozen, may be due to a lack of cleaning and maintenance of the system, which obviously would include the evaporator coils in the indoor unit and the condenser in the outdoor unit. Another simple reason may be given, for lack of adequate air conditioning in the room, for example by having a glass window without a thick curtain that prevents the entrance of the sun's ultraviolet rays into the room, creating the so-called greenhouse effect, in addition to cracks under the door of the room itself or of the windows that propitiate the entry of hot air from the outside. What originate all these examples ?, because they motivate that the team at a certain time of the year as in an intense summer, if we schedule the equipment to stop at a very low temperature, say 16 degrees Celsius, it will cost the system make it properly and therefore the thick return pipe would start to freeze, to the point if the system is somewhat coolant, which could damage even the compressor, with the arrival of coolant in the liquid state, fatal, for the flipper (valve ) of the compressor pump itself. A failure in the turbine fan, of the indoor unit, can also cause frost formation in the evaporator coil, because said fan does not reach the required speed and air flow through the evaporator. As well as damage to the temperature sensors. Or a break in the relay that feeds the compressor from the main electronic board, which will prevent the compressor and perhaps also the fan of the outdoor condensing unit from stopping at the time indicated by the central processor and the sensors, once Once the thermal load of the room has expired, it is therefore urgently necessary to replace said relay, as damage to the compressor can be irreversible. Luck. S @ lu2 friovega63.