first time that check engine light comes up at 110,000 miles.
does anyone know or have an idea where can I find the on board diagnostics reference book
or the mechanical/repair manual for the 2015 GLK 250 bluetec?
or someone who wants to sell it?

 

Hello and welcome to the forum! I believe you're able to purchase one from Haynes, who makes workshop manuals or download one online if you want them as digital files - there are a few options if you Google 'glk repair manual'