Hopefully you have kept records of all of your dealings with the dealer, and other mechanics you have consulted, so you can mount a case with Citroen to fix your car. If you haven’t done it already seek out an independent Citroen specialist and get their opinion on the cause of the problem. I would contact Citroen in writing, enclose all the documents you have kept, and request the car be fixed. Point out that while the warranty has expired you reported it while the warranty was current and that it wasn’t satisfactorily fixed. If necessary leave the car with the dealer until the cause is found and fixed to your satisfaction. Legal action is also an option, but I would suggest you pursue a direct resolution with Citroen before resorting to lawyers.