Most modern cars have to pass strict emission controls enforced by a country that has 24 aircraft carriers burning thousands of tonnes of toxic fuel every day. This patriotic country has the capability to make aircraft carriers and cars. Their cars, like their aircraft carriers, burn a lot of fuel. Most of it is wasted on poor inefficient engines designed for a 55mph speed limit. A European car company with autobahns and no speed limits could make a car do 200mph. To protect its home market the patriots tred to stop imports by putting in emission control laws, but the clever Europeans found a way around this law by fitting equipment that cleans up the emissions. This equipment can be very complex in a modern engine, but the most expensive part of it is the Diesel Particulate Filter (DPF). These require routine maintenance or replacement and can cost thousands. Yes, you read that right, thousands. As an experienced car checker I know by the smell of the exhaust if the car has emissions problems. This is backed up by computer diagnostics. A quick check is to smell the exhaust. If the fumes are unpleasant it could have a DPF filter past its best.