Vaillant ecotec plus 824 problems

I have vaillant boiler problems and would like some help on troubleshooting the issue.  I have a vaillant ecotec plus 824 and randomly the boiler does not ignite the burner.  I get the fault s.33 where the vaillant boiler manual tells me it is a problem with the air/flue system.  I have had a service guy check the boiler twice and the problem always comes back.  Any help would be greatly appreciated

Thanks

Ricky M

The problem with your boiler is the air pressure switch located on the top of your boiler.  Check out the parts diagram here.

This part has a diaphragm that has become blocked.  During normal operation when the fan activated air is sucked through the flue and a small amount fills the diaphragm and makes the electrical contacts within the switch.  In your case the flue is blocked and air is not inflating the diaphragm, your diaphragm has a hole, or your contacts are perished.  I would advise you get a corgi man to replace this part to ensure ongoing reliability of your boiler.