The following article covers a fault found with the Brother MFC665CW that causes an Unable to Print 51 error.
After some quick Google research I discovered that no one seemed to have an idea of what causes this fault and a number of forums claiming that a purge counter reset would fix it. Seeing that the purge counter causes an Error 47 (from memory) that seemed unlikely. An Error 51 is a problem with the cleaning unit and can be caused by paper or foreign objects getting stuck in the path of the carriage or the cleaning unit itself.
So some good old fashioned tech work was required. First off with all the covers, then I reconnected the scanner and keyboard in a way that allowed me to observe the printer and all moving parts during start-up. It was quickly apparent that the cleaning unit (right hand side) wasn’t getting drive. From there is was easy to trace the gears back to the cause of the problem.
As the picture shows, the gear that transfers the drive to the cleaning unit had split and was spinning on the shaft instead of providing drive. My easy fix was to use some high end hard plastics glue that also sticks to steel and glue the gear to the shaft. After the glue had dried for 30 mins – no more error 51.
I expect it would be possible to diagnose and perform this fix with out removing any covers. JUST BE CAREFUL NOT TO GLUE THE GEAR TO ANY BUSHINGS OR OTHER GEARS.
This is likely to be similar to other Brother printer models that take the LC57 ink cartridge (DCP130C, DCP330C, DCP350C, DCP540CN, DCP560CN, MFC240C, MFC440C, MFC465CN, MFC5860CN, MFC685CW, MFC885CW).