saunier duval sd620f

[meeper]

I have a problem with my boiler, when the hot water tap is on the water gets hot for about 30 secs, the boiler then cuts out and the water goes cold. About 30 secs later the boiler fires up again and the water then gets hot again, this cycle carries on whilst the hot water tap is on.
The heating does not come on either as the boiler keeps turning on and off.
If anybody knows the cause I would be grateful.

 

[weareleeds]

probably thermister i can't remember offhand whether it is the purple spot or the orange spot but it must surely have seen better days & be due replacement