Can you post the model number please.
Also are you saying the heater only comes on when you open the door?
Ok might be worth checking the door wiring harness if you havent already.
Remove the outer door panel and then remove the outer metal door panel from there.
There you will see the wiring running down the door to the underside.
Open the loom and check the wiring. Give each wire a tug and check for broken wires in there. As if the control board has been replaced as is the thermostat and the heater is working then its possibley a wiring issue.
As broke wires in the door on dishwashers are very common as they break in the loom so opening and closing the door can make and break contact with the wires in there.
So check the door wiring and see what you find and let me know.
No worries thats fine, Speak tomorrow.