Modern diesel cars are fitted with a Diesel Particulate Filter, commonly referred to as the DPF, which removes the sooty particulates from the engines exhaust emissions. These particulates are harmful pollutants, so the filters are a good idea but they can cause problems if they are allowed to become clogged. The filter is periodically cleaned of the sooty deposits by high temperatures in the exhaust generated by higher engine revs, typically generated at high speeds (e.g. during motorway driving). This process is called regeneration. If you’ve ever seen a gout of black smoke billowing from a cars exhaust when the driver puts their foot down, this is probably due to deposits burning off in the DPF. While some cars have active regeneration systems to counter a lack of high speed driving, this may not always work, particularly if you only do short journeys and stop start driving. It’s a good idea to take the revs up occasionally, when safe to do so, and if the DPF warning light shows you should follow the manufacturer’s directions for clearing the DPF. Ignoring the warning light could lead to a trip to the garage to have the filter cleaned or worse, replaced.