megane 2003- starter motor?

Not too difficult. New motor should be available for around £80 exchange I guess, and should cost about all in £160 fitted.
Have you checked the terminal connections to the starter motor itself? the spade terminals on Renaults I've owned had a habit of working loose and sliding off. Needed pushing back on from time to time. Cheaper than a new motor if this has happened.

ChicksFan
October 2009