How can I fix my valant ecoTEC exclusive 832 on F75 after changing sensor u?

how can I fix F75 on my ecoTEC exclusive 832. I have installed it 3 years ago. in the first 2 years it has failed and shown F22 almost every two month. I have been told by my plumber to pump up pressure to the system so, that have been doing, then in March 2013 it's has shown F75. I pump up the pressure again but it wasn't work any more. I called valiant engineer, they came and end up with changing sensor unit. Now I have F75 again i did pump pressure up to nearly full but still doesn't work. Any idea what was cause and how to fix it please.