no spark and pressure defferential switch does not open?

My vaillant turbo max 242E vuw boiler does not spark; the water pressure defferential switch does not open up have replaced with new one; boiler does not fire up. Air pressure switch appears ok; diverter valve ok;

If you manually lift up the defferential switch pin, the boiler sparks and fires up, but burners shut off after several seconds.

Please advise, I am lost. Tony 01753 570250; thelegaltable@tiscali.co.uk

Tony
September 2006