can anyone help. boiler has just started playing up. the burner is not modulating so the domestic hot water gets to hot and the overheat thermostat switches off the burner. leave the tap running and once the boiler has cooled down a bit the burner re-lights.(she who must be obeyed is not enjoying her shower anymore).The flow rate is exelent so the heat exchanger seams ok, I cleaned the DHW temperature sensor,   (it wasn't really dirty) the burner pressure is 1mb lower than spec. but it has been working fine (for about 5 years) I checked voltage at modulating coil on gas valve (burner lit) it showed 6 volts when burner first lit and then dropped to about 4 volts (burner still lit).Manual says change gas valve, which I'm happy to do if I can be sure that is the fault.If there is anyone out there who can advise it would be greatly apreciated.

update on my original question,I had a faulty lead on my volt meter,(now fixed) When I turn on the hot tap, with a volt meter connected to the modulating coil on gas valve, the voltage rises gently to a maximum of 9.5 volts, the burner ignites at about 3.0 volts, when the water temperature in the boiler reaches temperature (all led's alight) there is a click from somewhere, the voltage drops straight to 0 and the burner goes out. When a bit of cold water has passed through the boiler (1 led goes out) the burner re-lights .The boiler continues to cycle like this until the tap is turned off.(the heating circuit seems to be working normaly).The boiler doesn't show any fault indication and doesn't require re-setting.My guess is the voltage on the modulating coil should fluctuate and that causes the burner pressure to go up and down as required by demand, can anyone tell me what the electrical flow sequence is for the operation of the gas valve?What sensors tell the PCB to shut down the gas valve?

If its cutting out on the overheat then you should check the pump is working.

Have you tried the boiler with the heating on.

The rise in pressure is normal expansion.