Worcester 28i junior rsf air switch wont kick in

[arty21]

I have a 28i junior rsf(pre-condensing) The board fuse blue the other night so I replaced it and it blue again As I had come across this problem before on customers boilers I unpluged the fan from the board and the fuse said ok So I replaced the fan. The boiler fired ok but next morning the timer had switched it on but it had not fired. The fan and pump where running but the spark igniter had not operated. I tapped the air switch, nothing, I removed the red pipe and blue down it and replaced it and the boiler fired. I thought I had probably dislodged some muck and put it back together. The same evening the same thing happened. I remove the pipe and replaced it and the boiler fires. I replaced the switch. Next morning the same thing happened again. So new fan, new pressure switch and I have to remove and replace the red pipe from the air switch to get the boiler to fire. Once the boiler is working, if it cuts off on the thermostat it will fire again. If it cuts of on the timer then it wont work again. I am at a loss as to why. Can anyone help.
I am a gas safe registered engineer with 15 years experience in boiler repairs and yet this has got me puzzled.
Thank you

 

[lame plumber]

need to do some more posts and use the gsr forum away fm public viewing to get an answer I'm afraid

 

[Dotty]

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