Boiler firing on and off constantly

[boonboon]

When turning our hot water or heating on the boiler fires up for about 5 seconds. It then goes off for about 10 seconds and then fires up for 5 seconds again. This sequence happens constantly. Does anyone have any ideas what it could be?

I had this and the engineer replaced every single part, except the 'HT lead' because 'it never goes wrong'. We then got somebody from the manufacturer in and he replaces the HT lead and it has worked fine ever since.

What boiler do you have?

It's it actually firing and lighting? Does it fire for a longer time the first time and actually getting warm?
Could be anything really but you're likely to need a boiler engineer to fix it.

could be a pressure sensor. However I had one were there was a lose resistor on the circuit board. Difficult to find, easy to fix.