Locking out continuously

Danesmoor 15/19 runs for about 7 mins then locks out, restarts OK but every time locks out again, running for less and less time between lockouts. The engineer failed to turn up twice so will look for someone else on Monday, but in the meantime I'd love to get an idea of what the problem might be. Any thoughts gratefully received.

A pump failure or blockage would cause it to trip on its internal overtemp stat.

Presumably this is an oil boiler. Overtemp != lockout. Lockout is caused by flame failure for more than 10 seconds or so. I'd start by checking that any filters aren't clogged.

There might also be an optical flame sensor - if that is sooted up it could cause it sense flame failure.

No might about it, there is. A sooted up photo cell tends to produce an "all or nothing" failure mode, rather than progessively shorter run times.

On my Viessmann, the ionisation sensor was the problem. And they didn't mention the replacment which has worked perfectly - was a different design.