If you are considering a Peugeot or Citroen with the 2.2 HDi Diesel engine (or most other HDi models manufactured after around 2001) then you should know that these are fitted with a particulate filter (or FAP as it is sometimes referred) & this filter requires some expensive maintenance. The filter itself is located in front of the catalytic convertor & is intended to keep the emissions ultra low.
However, what isn't too widely publicised is the servicing that it requires - the manufacturers recommend replacement at around 75000 miles (or even sooner depending on the type of running that the car has done) at a cost of around £500 (+ labour) for a new filter. It can be replaced with a secondhand (cleaned) unit on an exchange basis for a much lower cost of £130 (+ labour), but this information is seldom volunteered by the dealers! What happens here is that your old filter is removed & sent to France to be extensively cleaned & added to the stockpile of exchange units. If the filter is neglected you will eventually see messages of UNBLOCK DIESEL FILTER on your on board computer which, if ignored, will eventually become ANTI POLLUTION FAULT & will put the car into limp home mode & unable to rev past 2000rpm.
The other expensive item is the Eolys fluid that is injected into the filter that is essential to keep the emisions low. It costs approx £20 per litre (only available from main dealers) & if this has been neglected & allowed to run dry will take 5 litres to fill & will also give an annoying DIESEL ADDITIVE MINIMUM LEVEL message on the on board computer.
After extensive searching through forums & other informative sources you will find that it is possible to clean your own particulate filter & fill the Eolys fluid yourself, or at a local garage that is familiar with the system, at a fraction of the costs that Peugeot & Citroen dealers charge, but you may find that the DIESEL ADDITIVE MINIMUM LEVEL warning stays & main dealers are very reluctant to get rid of this message for you unless carrying out the entire job themselves.
Very few sellers of these vehicles will actually volunteer this information in their descriptions so the onus is on you to ask the right questions before bidding or buying!