It could be numerous things. Rather than throwing parts at the problem based on other people's experience you really need a Lexia diagnostic (Lexia is the diagnostic equipment just for Citroen and Peugeot). It reads fault codes and will show live data.

Dealers charge a fair bit for this, but you'll find a network of Peugeot Citroen owners with the gear and will help you for a fraction of the cost.

Google "Lexia Owners Map" and click the link for frenchcarforum.com