Valliant Ecotec Pro no firing up?

I have checked the pressue in the tank is right, the expansion tank has 0.8 bar of pressure in it after system is drained and I have even replaced the water pressure sensor. It only fires up 1 in 30 times and then only for a few seconds.

When fitting a new pressure sensor do you need to fill it with a little water to stop an air lock or does the preessure in the system do that? Its the only thing I can think of.

Keep getting F75 reading "no pressure detected at pump" or something like that.

Help!!! Cold and starting to smell!!