DPF, AdBlue and EGR faults traced to the root cause

What does the clinic cover?

Diesel emissions faults on all the makes we look after, in three linked systems:

• Diesel particulate filter (DPF). It traps soot, which the engine burns off at intervals, called regeneration. If regeneration cannot complete, the filter fills up.
• AdBlue (SCR). AdBlue is added to the exhaust to cut nitrogen oxides (NOx).
• Exhaust gas recirculation (EGR). A valve returns some exhaust gas to the intake, and some engines also have an EGR cooler.

DPF root-cause diagnosis, AdBlue and SCR repairs, NOx sensor work and EGR valve and cooler work are done in-house.

Why do emissions warnings keep coming back?

Because clearing a warning does not fix what caused it. A regeneration burns off soot, but it does not change whatever stopped the filter clearing on its own. The systems also affect each other: an EGR fault can stop the DPF regenerating, and a failed EGR valve is one of the listed causes of SCR faults.

How do you find the root cause?

A fault code is not a repair, and a blocked filter is a symptom, not a cause. Depending on the car and the warning, we:

• read the fault memory
• read the soot load, the regeneration history and the pressure across the filter
• check the engine oil, including for fuel in the oil
• run a dosing test and check the NOx sensor readings
• test the EGR valve, and pressure-test the EGR cooler

Our DPF and emissions health check covers these tests with a written report. No work is done until you give us your written approval.

What does the repair involve?

• Fix the cause first, so the filter does not block again.
• Regenerate the filter in the workshop where that is what it needs.
• Replace only the part the tests prove faulty, with the filter as the last resort.

We then check the readings again and verify the repair, including a road test where appropriate.

Which page covers my car?

BMW and MINI

• DPF light: a DPF warning light or limp mode.
• AdBlue and NOx: an AdBlue warning with a mileage countdown.
• EGR valve and cooler: recall status checked first, then the valve and cooler tested.

Mercedes-Benz

• DPF: a filter that is blocked or not regenerating.
• AdBlue fault: an AdBlue system fault with a “start not possible” countdown.
• EGR valve: an engine warning light or poor running from the EGR valve.

Audi, Volkswagen, SEAT and Škoda

• DPF light: DPF, glow-plug and engine lights with loss of power.
• AdBlue fault: a no-restart countdown while the car still drives normally.
• EGR valve and cooler: EGR faults, and coolant loss with no visible leak.

Land Rover and Range Rover

• DPF and oil dilution: DPF warnings, limp mode or a rising oil level on the 2.0 Ingenium diesel.
• AdBlue, NOx and EGR: AdBlue countdowns, NOx sensor faults and EGR faults.

Do you remove DPF, AdBlue or EGR systems?

No. We repair them and never remove or disable them.

Is a DPF delete legal?

Almost never, for a vehicle used on the road. Government guidance on diesel particulate filters says removing a DPF will almost always make the vehicle illegal for road use. In the MOT, emissions equipment that is missing or obviously modified, and evidence that the DPF has been tampered with, are both Major defects.

The Road Traffic Act 1988 makes it an offence to use a vehicle that breaks the construction and use rules, and also covers altering a vehicle so that using it on the road would be unlawful. That can reach the person who did the work as well as the driver. The Department for Transport consulted in 2026 on making DPF removal illegal in every case.

Can the AdBlue or EGR system be removed or switched off?

Not by us. In the MOT, SCR parts and EGR valves are part of the emissions check, and emissions equipment that is missing, obviously modified or obviously defective is a Major defect.

Should I check for a recall first?

Yes. Check your registration on gov.uk. Recall work is done by the manufacturer’s network, not by us.