I am using one of the above air conditioning units, and have had the fault code E4 come up on the display. Does anyone know what this means?

I am a Technician in the Royal Air Force, and if I can find out what the fault code means then I can rectify this fault. I have searched the company website and am unable to find the required information, and tbh I also found this site with google so I thought I would give you guys a shout first,

Many Thanks

Simon