peugeot 406 coupe key fob wont work ?

You need to reprogram it to make it work. you can buy instructions on ebay for around £3. Alternatively, just google it. Try this link:
http://www.programyourkeys.com/Peugeot_406_Reprogramming_Key_Remote_Control_Plip_Instructions.html

The main man
March 2009

did the red light work on the fob before changing the battery ? if so the battery may be in upside down, or the wrong type please check

pug
November 2008