I have a xerox 5312/5214 Copier and it is showing a message of U4. In the manual it says to call for service. The machine is an older machine and I don't know how much it would cost to fix. Is there any advice you could give me as to what the problem might be?

The U4 code means you have a fser heat problem. The fuser is the assembly that heats up and "fuses" the toner onto the paper. It is located near where the paper exits the machine. There is a reset procedure you can use to try and see if the code will reset. If it does not then you would need to have it serviced. It is quite old and there is a concern wether it would be worth repairing. I would use www.partsdrop.com to get parts or more information if you need to persue a repair. I have used them quite a bit in the past.
Good Luck and Thank You for using Fixya.
Wheatland Services
Diagnostics:
To enter diagnostics: Turn on the copier while holding the '0' button...all control panel lamps will light.
To enter a code: Enter the first part of the code followed by the 'Start' button, then the second part of the code followed by the 'Start' button again.
Note: If an invalid code is entered, 'Er' is displayed. Press 'Stop' and enter a new code. *To clear a fuser code: Enter diagnostics, enter '52' followed by 'Start'. If a fuser code is current, the number '1' will flash, enter '0' to clear the code followed by 'Start' to lock in your selection. Press 'Stop' and then you can shut down.