On 26 Sep 2020 at 10:58:29 BST, "John Smith" wrote:

Vaillant Turbomax VUW boiler won't fire up - tries to but stops and
then on/off light flashing. Manual just says this indicates the
overheat themostat. Is this likely to be a failed part or something
bigger? I can't reset it from the controls.

Could it be a flame detection device? I haven't looked up what kind your
boiler has, but they are usually more-or-less easily replaceable.

--
Roger Hayter