I don't have an Intex tub; mine is by Bestway, and I've never been inside the cabinet. I have seen a YouTube video breakdown of a bestway, but not of an Intex, and a quick search only found one video of an Intex pump repair, but the breakdown part was not very helpful.
If it were me, I would open the cabinet up and start tearing it down to see what things look like on the inside. From what I've seen, I think there are two thermocouples, one on the intake and one on the output. There could really be multiple possibilities for the error message. If the water flow is blocked, the output might actually get over 104. There could be calcium buildup in the heater. There might be a broken or bad solder joint. The control board might be faulty. Or maybe a thermocouple failed.
I don't know whether I would be able to fix the problem if I worked on it, but I do know trouble shooting procedure, and I do know how to find info on the net if it is available.
Hope this helps, and good luck.