ultracom 18sxi f9 fault help (low pressure sensor fault)

[ascop]

hi my boiler wont fire tries 4-5 times to fire then displaye f9 as the fault which is a low pressure sensor fault
i have taken this off and give a gentle clean and have also cleaned out both hydro blocks and replaced still no joy
1 is it possible to bypass the sensor to check if this is the problem
2 any other possible causes

thanks in advance