Question

The pilot light on my Vaillant MAG125 will not stay lit. It was OK last weekend to begin with but then when I closed the water tap the burner did not shut off so I turned it off with the control on the front but now the pilot will not stay lit. I think the thermocouple has failed. What do you think?

Answer

You appear to have two problems. Yes the thermocouple is the first item to replace if the pilot light will not stay alight. The fact that the burner kept going when the tap was switched off will possibly mean replacing the water diaphragm as the increase in water pressure when turning off the tap is not closing the gas valve. The diaphragm is our ref. 117056. It is possible that there is a problem with the gas valve but the first step is to replace the diaphragm.