M on the dash means M for Manual mode is my guess. If the M is flashing then I assume that it's telling you to shift into the next gear??? Maybe you need to get your transmission serviced... The M is to indicate that you have changed for auto mode to manual only it does not indicate when to shift, if you don’t shift in time it will just hit the rev guard. When you say service what are we talking about, what is done in a tranny service? more detailed...