alpha 240E fault

[Faceache]

I have this combi boiler which has developed a fault on the central heating side. It fires up but fails to remain alight long enough to satisfy the heating demand.
After looking at the manufacterers fault finding flow chart I thought I had spotted the problem and suspected the central heating temperature sensor. After obtaining a new sensor I tried the boiler with the new sensor but did not fit it into the pipe work just to see if it would work. It did so I then drained down the boiler and fitted the new sensor into the pocket on the pipe work.
The trouble is it does exactly as it did before, failing to stay alight for anything more than a couple of minutes.
The hot water side of things is fine.
Does anyone have any experience regarding this model who could point me in the right direction?
Thanks in advance
David

 

[scatmanjohn]

You need to be able to measure how hot the boiler is getting when it cuts off.

Do you have any tools to do this?

 

[Faceache]

Thanks for responding Scatman.
Yes I've got the equipment to do that. I should have mentioned I'm more used to installing appliances so my fault findings a bit rusty.
Any advice much appreciated.
David

 

[scatmanjohn]

Its either getting up to temperature very quickly because there is a flow problem, either in the boiler or system (not sure if there is a return filter on that boiler).

Or its not getting up to full temp and turning off prematurely.

Measure the temp close to the ch sensor, see what it gets to when it cuts off.

 

[diy]

david

could be the ignition pcb