Good Morning, The model in question is: LG S12AA --- will be about 8-9 years old. I've been reading the forum for a few days, to see the possible causes of my AA not working in heat mode, The compressor does not start in heating mode, in cooling mode this summer worked well for me, and it still works fine, The compressor starts without problem. To also say that the plate was changed by insurance before summer, the AA indicated with blinks the fault, and he managed to get it through the insurance. since last winter it was not working in heating mode. What I have been able to check so far for what I have found in the forum and elsewhere, Sensors: I have checked 3 sensors of the plate that is in the compressor, I have looked for some tables that relate the resistance to the temperature. Sensor 1- one that goes inside a tube (for the values that it gives me must be the one of discharges), I have disassembled and tested it at different temperatures and mark according to the table. Sensor 2- is the sensor that is just behind the grid (I suppose it will be the Air) I have measured it at room temperature, and it marks according to the table, right on the same connector, there is another sensor, I can't see where it goes, since it gets behind the tubes down, but mark at room temperature according to the table. I have a device similar to this mounted in another room, and I have checked the values of sensors 1, 2 and 3 at room temperature and they mark the same. The solenoid valve coil gives continuity and is heard when you make the change of cold-heat mode. Right now I don't know where to go. I just read that in the Split there is another sensor that measures the temperature of a battery, but I'm not sure where it is, can this be the cause? Do you know how to access this sensor? Could it also be that the valve is stuck and does not change to heating mode? Any help will be welcome. Thank you

Good day. It may happen that some of the refrigerant is missing too, since in cold mode you think it yields but in heat mode until it does not reach the necessary psi the equipment does not turn on. Luck

Check sensors, calibrate them and check the microcontroller, if not change universal card.