Problem with vaillant turbomax vuw282

[Gary80gas]

Just looked at an old vaillant turbomax. Set the boiler calling for heat and burner dosent light. So removed front cover and instantly sparks and lights for 30 seconds then goes out a few seconds later repeats lights 30 seconds and goes out.
The pumps ok and the fan runs, I pulled fan apart to check for obstructions blew through to venture from aps all seems clear. Checked continuity on heat stat all ok. Cleaned spark electrodes. Now its lighting for 5 seconds and going out cycling with 20 sec gaps. Im assuming its the air pressure switch as it lit when I opened it up to the air in the room. Ive got myself confused as every time I pull it apart it behaves slightly differently. I still suspect the aps. Any ideas or advice appreciated. Ta.

 

[AMGasServices]

From past experience before changing any component give vaillant a call the have a diagnosis program that will indicate when the aps makes and breaks the circuit.

 

[Last Plumber]

You need to look at air pressure issues:-
Seal, flue (in and out), fan, heat ex, venturi, air pressure switch, tubes etc.
Along with Gas pressure on min and max rate. 2mb - 10.6mb.
Start with those things and you'll most likely sort it.

 

[Gary80gas]

Finallly got it solved. When I returned the pump and fan ran but now not even a spark. I rang vaillant and they wanted me to replace circuit board because fan light on front was not coming on. This didn't make sense. I went through the whole flue system again. I measured pressure difference on venturi and got 1 mb. I mention this to vaillant not a problem to them. I cleaned it all again changed pressure switch to be sure and all fired up nicely. Venturi pressure back up to 5mb even though I didnt find any obvious problem.....