intermittent lock out

[sgtian]

Hello, ive been to a greenstar worcester bosch combi , with intermittent lock out , I've traced it back to the flue, with the burn air intake pipe pipe removed the boiler will not lock out , but when it is fitted it lockds out. now that shows me that the air intake side of the flue is either blocked or the seals have gone on the flue extensions.
How do i prove that the seals have gone? with out taking it apart.

 

[SimonG]

Measure flue gas readings on inlet air at burner.

 

[SimonG]

Boiler could be set up incorrectly on the air side. What's the excess air %