Good work! That is the full schematic, before all the important information was removed. I can now see that it is not very well designed!
Can you make some voltage measurements on the main microcontroller U7. The 0V reference for the black multimeter probe is the GND rail, which is the tab of the 7805 regulator.
Pin 14 is the +5V power supply to the MCU. It needs to measure +5V for the MCU to operate.
Pin 4 (MCLR) needs to be high (+5V) for normal operation.
Pin 3 should have a square wave at mains frequency. If you have an oscilloscope you can check this. If not, just check that the voltage is around 2~3V.
Pins 15 and 16 connect to a 4 MHz crystal which sets the internal operating frequency of the MCU. It is not easy to test this signal because even a small amount of loading can stop the oscillator. You MAY be able to see oscillation on one of those pins using an oscilloscope, if you have one.
Pins 2, 9 and 18 connect through to the remote. It looks like they are all inputs that are driven by the remote, but I can't be sure. Check that the voltages on those pins change when the relevant button or switch on the remote is changed. Make sure the remote is set correctly to allow the heater to operate.
Pin 17 is a signal called "THERMAL" which (presumably) represents a temperature. The voltage ultimately comes from the two-pin connector called CON1, which I think is the two screw terminals on the left hand edge, labelled "TERMINAL". I'm not sure what type of temperature sensor is supposed to be connected here. You should compare the THERMAL voltage between the faulty board and a good board, with no temperature sensor connected and with a normal temperature sensor connected.
If you haven't already, try swapping U7 from a good board into the bad board. (After checking that the +5V supply rail is correct.)
Can you check those voltages, and provide information on the temperature sensor and the remote. A photo of each could be helpful too.
