I have a xerox 5016 copier ,

On this model sometimes the plugs on the connectors go bad.
They arc and will not connect good. Sometimes the ends have to be clipped and new pins male and female have to be crimped back on, replacing them on the machine part wiring connectors and fuser rod ends. this fuser kit is a part that is availiable at xerox.
A bad thermistor can cause this type of problem. If the machine overheats, the fuser triac pack can cause over heat problems as well.
On overheats the fuser fuse needs to be replaced. Do not replace with nothing else but a Xerox fuse, this is a safety device. Very important.
Otherwise place a service call to have this area repaired by xerox.

There are a few U4 codes on that machine, I am going to guess you have a U4-6, which means the fuser web life is exceeding its count. What you want to do is turn the machine off, then press and hold down the "0" thats the zero button while you turn the machine on. You should get a number like r5,r7 or r9 on the screen. Next enter the number 10, then press start, then the number 61 and press start. If the number is 50 or higher there is your problem. Set it to zero and press start. Then turn the machine on and off. If that clears it, you want to then look at the fuser web which is under the black cover over the fuser, It should have caution labels on it concerning it being hot. There are 2 phillips screws on the ends of that assembly, remove them and see if the web is pretty dirty and at the end of the shaft, pretty much run off the shaft. If it is it needs replaced.
U4 can also mean the over heat is blown, that is under the web assembly , it is a white block about 2 inches long with 3 wires attached. You would have to have a meter to measure whether is it good or bad.