Modern diesels are clean, economical and great for long journeys. But many owners who mainly drive short distances meet the same two problems sooner or later: a blocked diesel particulate filter (DPF) and a sticking exhaust gas recirculation (EGR) valve. Understanding why they happen helps you prevent them, and knowing which parts are involved helps you fix them properly.
What the DPF does
The DPF sits in the exhaust and traps the soot that diesel engines produce. Over time it fills up, so it needs to burn off, or regenerate, the soot at high temperature. This happens in two main ways:
- Passive regeneration happens naturally when the exhaust is hot, such as during steady motorway or A-road driving.
- Active regeneration is triggered by the engine control unit, which adjusts fuelling to raise the exhaust temperature when the filter is getting full.
Active regeneration needs the car to keep running at a steady speed for a while. If the journey ends before it finishes, the soot keeps building up.
Why short trips cause DPF problems
Short, slow journeys don't get the exhaust hot enough for long enough. Each interrupted regeneration leaves more soot behind. Eventually a warning light comes on, and if it is ignored the car may go into limp mode with reduced power. A badly blocked filter can need a forced regeneration with diagnostic equipment, cleaning or replacement.
DPF warning signs
- A DPF warning light on the dashboard
- Cooling fans running after the engine is switched off
- A higher idle speed or a hot, acrid smell during regeneration
- Reduced power or limp mode
- Higher fuel consumption
When the DPF light first appears, the handbook usually recommends a steady drive at a moderate speed to let the filter regenerate. Check your handbook for your car's exact advice.
What the EGR valve does
The EGR valve feeds a small amount of exhaust gas back into the intake. This lowers combustion temperatures and reduces nitrogen oxide emissions. Many engines also have an EGR cooler to cool the gas first. Over time, soot and oil vapour build up in the valve and the intake, and the valve can stick open or closed.
EGR warning signs
- An engine management light, often with EGR flow or position codes
- Rough idle, hesitation or flat spots
- Black smoke under acceleration
- Reduced power and limp mode
- Poor fuel economy
EGR and DPF problems are often linked. A faulty EGR valve can increase soot production, which fills the DPF faster.
The genuine parts usually involved
A fault light doesn't always mean the DPF or EGR valve itself has failed. Often the cause is a sensor or a related part:
- Differential pressure sensor: measures how blocked the DPF is. A faulty sensor or a split pipe can give false readings.
- Exhaust temperature sensors: help control regeneration.
- EGR valve and EGR cooler: can stick, leak or clog.
- Gaskets and seals: exhaust and EGR gaskets should be replaced whenever parts are removed.
- Glow plugs, injectors and thermostats: faults here can increase soot or stop the engine reaching temperature.
- AdBlue system parts on newer engines, which work alongside the DPF to meet emissions rules.
Make-specific points
Volkswagen Group TDI, Ford EcoBlue and TDCi, and Hyundai and Kia CRDi engines all use a DPF and EGR system, but layouts differ. Some engines have high- and low-pressure EGR circuits, and sensor positions and part numbers vary between engine codes. Our guides to using your VIN and reading exploded parts diagrams will help you find the right part for your engine.
Prevention tips
- Give the car a longer, steady run every week or two if you mainly do short trips.
- Use the engine oil specified for your engine, as the wrong oil can increase ash in the DPF.
- Fix engine faults promptly, especially misfires, injector issues and boost leaks.
- Don't ignore the DPF light, as early action is often all that is needed.
A note on removal
Removing a DPF or disabling the EGR system is illegal for road use in the UK and will cause an MOT failure. It is always better to repair the system properly with the correct parts.
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