Man Neoplan 2010. Hispacold A/C system

hi there have Man Neoplan coach with Hispacold A/C system keep getting fault code E33 which i think is related to low pressure. This fault keeps apearing on a regular occurance even though the gas level is fine and the pressure switch wireing is ok. the coach has been in to several workshops all over europe & still no fix. I have herd of this happening on other coaches of the same make.
has anyone got any ideas thanks
ps working on coach blind as i havent got any scematics

now fixed there was broken earth wire at compressor clutch thanks for all the replies