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What a DPF does
A diesel particulate filter sits in the exhaust and traps the soot a diesel engine produces, stopping it coming out of the tailpipe. Over time the filter fills up, and the car is designed to burn that soot off by heating the filter to a very high temperature. This is called regeneration. Problems start when regeneration can't happen often enough.
Why DPFs block
- Short journeys and city driving. Regeneration needs a sustained period of steady driving to reach temperature. School runs, short commutes and stop-start traffic around Birmingham often don't provide it, so the soot keeps building.
- Interrupted regenerations. Switching off partway through an active regeneration means it has to start again next time.
- Underlying faults. A faulty differential pressure sensor, a sticking EGR valve, worn injectors, a turbo leaking oil or a thermostat stuck open can all stop regeneration working or make the engine produce more soot.
- The wrong engine oil. DPF-equipped diesels need low-ash oil. The wrong oil leaves more ash in the filter, and ash can't be burned off.
Signs of a blocked DPF
- The DPF warning light is on
- Noticeable loss of power, or the car going into limp mode
- Higher fuel consumption
- The cooling fan running after you switch off, a sign the car was trying to regenerate
- An acrid, hot smell from the exhaust
- The engine management light coming on alongside the DPF light
How regeneration works
- Passive regeneration happens naturally on longer, faster runs, when the exhaust gets hot enough to burn soot without help.
- Active regeneration is triggered by the engine computer, which adjusts fuel injection to raise the exhaust temperature. It needs you to keep driving for a while to finish.
- Forced regeneration is run with a diagnostic tool while the car is stationary. It's how we clear a filter that's too full for the car to manage on its own.
DPF light on: keep driving or book a check?
An amber DPF light doesn't always mean the filter needs professional cleaning or replacing. It can mean the filter hasn't finished regenerating. With an amber light and no other symptoms, your handbook may describe a drive that lets it regenerate. Follow the instructions for your exact make and model, because no single speed or time suits every car. The RAC's DPF guide explains more.
Get it checked if the light stays on, comes back quickly, appears with other warning lights or comes with a loss of power. Don't try a long drive if the car is in limp mode, overheating, smoking heavily or showing a red or flashing warning.
Cleaning, regeneration or replacement?
"DPF cleaning" can mean different things, so it's worth asking any provider exactly what they'll do. A regeneration on the car and removing the filter for specialist cleaning are different jobs. No treatment should be promised as a cure before the car has been diagnosed, so we'll tell you what we've found and what we recommend before you agree to anything.
| Option | When it works | Limits |
|---|---|---|
| Forced regeneration | The filter is loaded with soot but otherwise healthy | Won't fix the cause if a sensor or engine fault is to blame |
| Chemical clean plus regeneration | Heavier soot build-up that a regeneration alone can't shift | Can't remove ash, which builds up over very high mileage |
| Replacement | The filter is cracked, damaged or full of ash | The most expensive option, so it's the last resort |
Before cleaning anything, we read the fault codes and check the pressure sensor readings and related components. Cleaning a filter without fixing a faulty sensor or EGR valve just means it blocks again. If the filter really does need replacing, we'll tell you honestly rather than sell you a clean that won't last.
Can I just remove my DPF?
No, and we don't offer DPF removal. Driving on UK roads with the DPF removed is illegal, and it's an automatic MOT failure, since MOT testers check that a DPF is fitted where the car was built with one. Government guidance puts the fine at up to £1,000 for a car and £2,500 for a van, and it can also cause problems with your insurance. A properly cleaned, working DPF is the legal fix, and usually a far cheaper one than people expect.
How to keep your DPF healthy
- Give the car a steady run of 20 minutes or more at dual-carriageway or motorway speeds every week or two, especially if most of your driving is short trips
- If the DPF light comes on, don't ignore it. Following your handbook's advice early can often clear it before it needs cleaning.
- Use the low-ash oil specification your manufacturer requires
- Get warning lights and engine faults looked at promptly, because many of them stop regeneration working
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Where we work
We're based in Small Heath, Birmingham, and come to you across the West Midlands, including these areas:
Frequently asked questions
What causes a DPF to block?
Mostly short journeys and stop-start driving, which don't give the filter enough heat to regenerate. Faulty sensors, EGR valves, injectors and the wrong engine oil can also cause it, so we check for those before cleaning.
Is DPF cleaning cheaper than replacement?
Usually by a long way. A filter that's blocked with soot but otherwise sound can normally be cleaned and regenerated for a fraction of the cost of a new one.
Can a blocked DPF fail an MOT?
A missing or tampered DPF is an MOT failure, as is excessive smoke. A blocked filter often brings on warning lights that can also cause problems at the test, so it's worth clearing it first.
Do you remove DPFs?
No. Driving on UK roads with the DPF removed is illegal and fails the MOT. We clean and regenerate the filter and fix the underlying cause instead.
Will the DPF light come back after cleaning?
Not if the cause is fixed and the car gets enough steady driving. That's why we diagnose why it blocked, not just clean it.
Can I keep driving with the DPF light on?
With an amber light and no other symptoms, your handbook may describe a drive that lets the filter regenerate. Get it checked if the light stays on or returns, if other warnings appear or if the car loses power. Don't try a long drive in limp mode, or with a red or flashing warning.