Fuser unit failure

I agree the fault is almost cetainly with the fuser unit.

This code means that you have a failure of a part in your fusing unit. The fusing unit is the assembly that gets hot and "fuses" the toner onto the paper. It is located right where the paper exits the copier. The most common part failure in this unit is the thermal fuse. As it's title suggests this part is a fuse which is suppose to blow if the fuser overheats. They usually just fail due to age. There are 2 other possibilities although not nearly as common as the thermal fuse. The other 2 possible failures are the thermister which reads the temperature of the unit and the heater lamp which actually provides the heat.
The thermal fuse is a cheap part at around $5.
You will most likely need a service person to come out and diagnose the problem and repair it. Unless you are mechanically inclined and have a service manaul you might have a hard time getting the fuser out of the machine and dis-assembled in order to replace the parts. There is also a specific reset procedure that must be followed in order to clear the C0500 from memory after repairs are made.
Good Luck and Thank You for using Fixya.

This problem is fuser (the section that gets hot) related. It could be one or more of the following things :
Thermistor failure
Heater lamp failure
MCU PWB failure
Thermostat failure
Power supply failure
Interlock (door) switch failure

It could be that the thermistors(monitoring temperature) are just dirty and need cleaning or the thermostat has blown. THe heater lamp inside the fuser roller may have stopped working or their is a power supply issue to the fuser.
You could try pulling the fuser unit out of the copier and checking it / clean it. With these models its usually easier and cheaper to buy a whole new fuser unit.
You could try resetting the fuser error code in service mode.
To enter the service mode press the following keys
Clear (then) Density select (then) Clear (then) Density select (while in this mode do not adjust anything else as this could further damage your system)
enter "14" (with the copy quantity keys) and press copy key. Now restart the copier.

This is most probably a failure in the Fuser assembly heater element.
The fuser is the unit in the back of the machine where the hot rollers sit.

If you are not competant at fixing electronic equipment then you will need a service technician to fix this. High voltages are present at the fuser.

This is the fuser unit that does not reach working temperature, you will need to replace the fuser unit.

Brother error e50
The error code E50 indicates a fuser unit failure - the fuser unit is no longer working and needs replacing. The error may also display itself on your Brother printer's front display panel as "Fuser Unit Failure" or "Replace Fuser Unit."

What Does the Brother Printer Code E50 Mean? - Small ...

E004 Fixing Unit Failure.
I would recomend calling repair Tech to resolve this issue....unless you know how to access service mode and perform a Printer Reset operation. Could be complete Fuser Unit failure.

In the fuser unit the Heat roller thermister indicates a low temperature error. Replacing the fuser unit will usually resolve this. Fuser units are consumable items bur for an early life failure such as this it may be worth contacting the manufacturer.

More than likely you have a fuser failure.
either contact canon or whoever you purchased the machine from.
The most likely solution will be to replace the fuser unit.

The printer has a Reset mode- somewhere. But different makers/models have different settings. Try the manual and if not mentioned there try the Canon website. Failing that try this- with the printer switched off press a combination of 2 buttons[any 2] with them pressed switch the printer on did it reset?- then try another combination. take a look also at www.fixyourownprinter.com also here http://www.fixya.com/support/p360551-canon_pixma_mp780_all_in_one_inkjet