i guess when it goes on to limp mode the fault codes stored are dpf related if so yes replace it or have it mapped out best way for ward, as for clean it there ar professional company's out there that can clean them correctly, a pressure washer and a tub of truck wash wont do it correctly within a few thousands miles you will be back at square one again ,,its not clear what year your car is if its pre 2010 its psa 1.6hdi engines the filter for these are fairly cheap £115,, the same engine is used in volvo and ford cars and these dealers strictly say its a service item and has a 75.000 miles limit on them,, they also have a additive bag that needs refill and rest every 45.000 miles ,