from what i can see of the diagram there are 2 switches in the heater lines
to check both at once the best way i can see is to see if you have 240v across board terminals 1 and 2 also poss label N and RS wires N is white and A is brown and should be terms of heater relay
if you get 240 then the board is not turning heater on
if you dont its in the pressure sw circuit
from the temp sensor reading and the little info i found it sounds about right
Dave