I'll try to go into as much detail as I can and explain what I did and why. If there are any questions, don't hesitate
First, I tried to guess what's wrong from what I can see without opening the device.
I know the controller works since it turns on, has a working display and the buttons also work.
I know the motor and heater are working, this means that the power supply section is also fine.
The biggest clue was the temperature itself. while 32F seems arbitrary, 0C can't be a coincidence. I suspected that it wasn't just stuck at some temperature, but displaying the default value when not getting a valid reading.
This means either the thermistor itself is faulty or the reading is not getting where it needs to go.
Then I opened it to have a closer look and figure out what everything does.
There are 3 metal tubes with wires coming out of them, these are the temperature sensors and the water level sensor, but which is which?
One tube has 3 wires coming out of it and is not touching anything. Two of the wires are green, these are the wires for the thermistor (this makes sense since thermistors don't care about polarization, so they usually have the same color wires). The extra wire is connected to the tube itself and is part of the water level sensor.
Another, shorter, tube has only one wire coming out of it and the tube itself is also not touching anything. The wire is connected to the tube itself - This is the other part of the water level sensor. When the water reaches the shorter tube, it shorts the two tube bodies together, thus indicating to the controller that the water has reached the desired level. (This means that this wouldn't work in distilled water, good to know).
The last tube has 2 wires and is clamped to the heater. The wires are the same color (red) so I suspected that this is another thermistor, but why two?
After following the thermostor wires and the traces on the board, I saw that the red wires were connected to the same board as the green wires, but the red wires' traces on the board stay on the board and lead to part of the power circuit. The green wires lead to a connector, from that they go to another board via a flat ribbon cable and then to some component on the second board. (There was also a small board which had the connectivity hardware that had to be removed in order to reach the components under it)
This told me that the red-wired thermistor is not read by the controller and is only used for protection if the heater gets too hot.
The green-wired thermistor is the actual water temperature sensor.
After I was confident I understood how everything comes together, I started troubleshooting.
I measured the resistance of both the thermistors and they came out about the same and a value that makes sense for a working thermistor (can't rememeber the specific value, it's in the KOhms range).
To make sure that the problem isn't with the thermistor itself, I swapped the thermistors. I connected the green wires instead of the red and vice-versa and powered the device. No change, still 0C and the motor and heater still work. So both the thermistors are fine. (BTW, in order to test the heater and motor, I shorted the two tubes with a screwdriver to trick the controller to think there's water between them)
This led me to suspect the component that the thermistor is reaching (I followed the traces as I mentioned, and checked continuity with a meter and it seems fine up to the component)
I also followed the traces from the component to the processor and they seem fine too.
I checked what this component does and google revealed it's an ACD. AHA! that makes total sense.
I downloaded the datasheets for both the ADC and the processor and made sure the pins were connected to where they should - and they were.
I desoldered the ADC and resoldered it to the board to make sure it's not just a cracked solder joint or a short, but it didn't help.
I also tried cleaning all the boards with alcohol and an ultrasonic cleaner to eliminate the possibility of particles causing a short. It didn't help also.This led me to the conclusion that most likely the ADC is dead.
I thought about probing the ADC and seeing what it outputs, but the datasheet for it was not very helpful and mostly in Chinese, so I decided to replace it since it's very cheap.
After the component arrived (a month later), I swapped the ADC with the new one and tested again - SUCCESS!
Put everything together, tested again and started cooking.
Here are more photos of the inside: