1993 500SEL odometer reset
 

Hello,

After having pressed the car's odometer reset button it no longer works.

Any idea what would cause this problem and, what's the degree of difficulty of removing the odometer?

Thank you very much in advance.

Regards,

Orazio

You broke a gear tooth or two off one of the odometer gears. This happens a lot. First rule of thumb: Don't reset while moving, ganted that the gears are flexible to some degree, they are 20 + years old and more brittle than when new, hence, they break teeth.
The cluster has to come out and the speedometer has to come off. there are repair gear sets available online.

This is where to buy the gear set and they have good instructions to replace the gears. I just used them a couple months back.
http://www.speedometergears.com/

Instructions for removing the cluster abound online , do a search on this forum and in the DIY section. Pretty straight forward, but scary the first time you pull the cluster. Make the tools from a stiff coat hanger and save the tool bucks for the gears.