Potterton Netaheat 80e - diagnosing air pressure switch problems?

Hi,

I have a Potterton Profile Netaheat 80e boiler. I had some problems with it last year and through these pages I traced a badly soldered PCB joint that, after repair, fixed the problem.

Recently, however, I found that the fan was starting up but the spark never came to light the pilot and the on/off cycle of the fan would continue indefinitely. Reading these pages it seems that the air pressure switch is the likely culprit. So far, I have checked the power coming from the three terminals and only one (the white) is live and I don't hear the tell-tale 'click' just after fan starts. I've checked the pipes connected to the pressure sensor and these look fine and well sealed. The fan is blowing out of the vent fine and I can feel a small air flow from the red pipe when disconnected from the pressure unit. I have recently resorted to the hair dryer trick mentioned elsewhere on this site and that has got the thing started a few times. It has sometimes gone fine for a few days but in the past few days it has needed starting via the dryer method every time. I'm not sure what the dryer is doing exactly (ie, which part of the unit is sensitive to the heat) but I think it's getting less responsive as time goes on (as well at being a pretty inconvenient fix).

My next move was going to be to try replacing the pressure unit but is there anything else to try before this? Is there any test that can be performed to confirm that the unit is the problem?

Many thanks for any help that you can give. Now, back to the hair dryer...