I have a fault on fan or flue on my baxi combi, 105e ?

I have a fault on fan or flue on my baxi combi, 105e, engineer has been out, serviced boiler, cleaned out venturi in flue and checked all piping for blockages. Fan is still running perfect as before and pressure is still on a good 1 bar. But, still keep getting fault on fan or flue. Seems strange but it seems to be mostly when outside temperatures are very cold, e.g in the night. Does anybody have any ideas?