Diesel Particulate Filter (DPF)

If you're buying a new car and plan to use it mainly for town-based, stop/start driving it would be wise to avoid a diesel car fitted with a Diesel Particulate Filter (DPF) because of the possible hassle of incomplete 'DPF regeneration'.

How do they work?
Diesel Particulate filters (DPF) or 'traps' do just that, they catch bits of soot in the exhaust. As with any filter they have to be emptied regularly to maintain performance. For a DPF this process is called 'regeneration' – the collected soot is burnt off at high temperature to leave only a tiny ash residue. Regeneration is either passive or active. Passive regeneration takes place automatically on motorway or fast A-road runs when the exhaust temperature is high.

During active regeneration you may notice the following symptoms:
• Cooling fans running
• Increased idle speed
• Deactivation of automatic Stop/Start

If you ignore the DPF warning light and keep driving in a relatively slow, stop/start pattern, soot loading will continue to build up until around 75% when you can expect to see other dashboard warning lights come on too. At this point driving at speed alone will not be enough and you will need to take the car to a dealer for 'forced' regeneration.

What can prevent normal regeneration taking place?

• Frequent short journeys where the engine does not reach normal operating temperature
• Wrong oil type - DPF equipped cars require low ash, low sulphur engine oils
• A problem with the inlet, fuel or Exhaust Gas Recirculation (EGR) system causing incomplete combustion will increase soot loading.
• A warning light on or diagnostic trouble code logged in the engine management system may prevent active or catalyst regeneration
• Low fuel level will prevent active regeneration taking place. As a general rule ¼ tank is required

If you continue to ignore warnings and soot loading keeps increasing then the car won’t run properly and the most likely outcome will be that you will have to get a new DPF. DPFs are designed to last in excess of 100,000 miles.