thermal store cold

Can You help please , I have been to a call out on a pulsacoil A class, and the thermal store is not heating up. If i press the boost button i get 240v to the immerion heater but it only stays on for about 1 min then drops out. I took all the sensors out of there pockets but the immersion wouldnt come back on =, it seems to be the PCB is holding it off. How can i tell if it is a faulty PCB or a sensor. I have checked everything else ie pump is running , header tank is full immersions are ok and no fault lights on the panel.

Thanks Chris

Right;
Have you the book?
I would say no.
Down load the book from Gledhill which is free.
Then you can go through the fault codes