Malfunction(Code 3451) of Bizhub C252

This is a fuser heat error. The fuser unit is located in the area where the paper exits the machine. It heats up and "fuses" the toner onto the paper. The error code means that it did not heat up properly. This means you will most likely need a service person to come out and diagnose the problem.
You can try to reset the error but I doubt it will help.
Turn off the MAIN power switch (not the one on the control panel)
Now hold down the utility key while turning on the main switch and continue holding the utility key for at least 10 seconds. You should see a screen come up with nothing but "trouble reset"
Touch trouble reset and you should see a small "ok" come on. Then turn off then on the main switch again.
Good Luck and Thank You for using Fixya.

This is normally an issue with the fuser unit however if you replace the fuser unit and still have the issue it most likely will be the prcb there are ice fuses on the prcb board that blow you should replace the board if that fails then you will need to replace the dc power supply unit
hope this helps