Xerox 1025 copier Problem

Does anyone know what status code U4 signify? I turn the machine on,
it says "wait" while warming up and then shuts down with status code
U4.

It has been working fine up until today. It would go into status code
U4 whenever there was a delay in copying something. Is it a "sleep"
function? I would turn off the machine and turn it on again and it
would work just fine until today.

Thanks in advance

Joe
.