BAXI COMBI 105 HE - Intermittent fault

[akgoddard]

Please help. I have a problem with my Baxi Combi 105 HE.
There seems to be an intermittent fault which leads to the hot water (and central heating – the symptons are the same) only coming through luke warm. When hot water is turned on, the burner lights but then cuts out after around 8 seconds then restarts a few seconds later. The temperature does not go up above around 40 degrees (sometimes 30, sometimes 50).
I called a heating engineer out and he first of all replaced a split diaphragm and then the DHW temperature sensor (though in hindsight this couldn’t be the cause sine the fault is on the CH system also). He then tried a new PCB though this did not fix it.
As I say the fault is intermittent so sometimes the hot water and CH are fine and temperature goes up to around 80 degrees as I would expect.
This intermittent fault started some months ago but only occasionally did not work – now it has around a 10% success rate – so I am keen to get this sorted!
Not sure if this is a burner problem (gas valve? Gas pressure? But then why would the fault be intermittent) or a control issue (leading to burner cutting out at too low temperature – but, to me, this now seems unlikely given that PCB replacement did not fix and HW sensor has been replaced).
Really appreciate any advice?
Andy

[htg engineer]

Could be many things - but throwing parts at the boiler (which your last 'engineer' has done) is not the answer or cheap.

It could be the gas valve, you can get lazy/sticking gas valves, but it's impossible to diagnose faults like this without seeing the boiler. The cheapest is to replace the CH sensor as this controls the temp of the heating/boiler temperature so can cause problems with both CH and DHW.

htg

[akgoddard]

Was the air pressure switch - central heating and hot water back :-)