Modern diesel cars have a DPF in the exhaust that traps soot.
The filter needs to burn off soot regularly, a process called regeneration.
How regeneration works
At sustained higher speeds, exhaust temperatures rise enough to burn off soot. Some cars also inject extra fuel to trigger regeneration.
Why DPFs clog
- ▸Lots of short, slow trips
- ▸Interrupting regeneration by switching off
- ▸Wrong engine oil
- ▸Engine faults causing excess soot
Warning signs
A DPF warning light, reduced power, or the engine fan running after switching off may indicate a regeneration in progress or a blockage.
Prevention
Drive at steady motorway speeds for 20 minutes or so every couple of weeks if most trips are short. If you mostly drive short trips, a diesel may not be the best choice. See our short trips guide.
If the warning light comes on
Many manuals advise driving at a steady speed above a certain level for 10–20 minutes to allow regeneration when the DPF light appears. If the light stays on or other warnings appear, a garage can carry out a forced regeneration using diagnostic equipment, or clean the filter. Replacement is expensive, so act early. Removing a DPF is illegal in many countries and can cause the car to fail inspections.
CAUTION: This is general automotive information, not professional mechanical advice. Always follow your vehicle manufacturer’s specifications and service schedule, and have brakes, steering, tyres, and other safety-critical systems inspected by a qualified mechanic.