Potterton Puma 80e - low pressure switch - boiler not firing?

I have just changed a low preesure switch on a Potterton Puma 80e. When I recconnect the two cables the boiler does not fire up. If I leave the cables off the boiler does fire up but then the pressure increases. I have taken the pressure switch off and cleaned inside and made sure there is no blockage before fitting the new pressure switch. I have tried releasing air on pump but no joy.Do not know what else to check as the cables must be connected but then I am left with no DHW or CH!!..please help.