potterton puma 80?

i have a potterton puma 80 combi boiler that is making clunking /issing when trying to use the C H it will come on for about 30 seconds and knock itself off along with the pilot light it is o.k. on hot water. upon taking the front cover off i noticed that there is a red light on on the pcb board could it be the dhw switch sticking? thanks for any advice chris