I think I've had a similar problem in a Samsung dishwasher. It didn't heat the water and the display returned a low temperature error. I took it all apart, tested the heater, desoldered and checked the relays - it was a PITA to pick the gel off the PCB. The relay coil circuit went through a pressure sensor made of a diaphragm and a micro switch. All of these things seemed OK, but then it struck me. The washing machine must be sure that the heater is filled with water and there is sufficient flow - that's why there is a pressure sensor at the heater inlet. The real problem was the PUMP! Of course, I've checked it before - it ran (kind of), but not good enough. It had a shorted winding, or something - it ran too slow to build up required pressure, and it got hot after 30s. The heater was disabled by the micro switch in the pressure sensor. I've replaced the pump and now it runs well.