The tester has found that the DPF fitted when the car was built is gone, or that the casing has been cut open and welded back up. Since 2014 this is an automatic MOT failure on any diesel that left the factory with one — it isn't a judgement call and there's no way around it.
Wording as it commonly appears on an MOT certificate: “Diesel particulate filter missing or has been obviously modified”.
What does it mean?
The tester has found that the DPF fitted when the car was built is gone, or that the casing has been cut open and welded back up. Since 2014 this is an automatic MOT failure on any diesel that left the factory with one — it isn't a judgement call and there's no way around it.
How serious is it?
Usually recorded as: Major. The tester assesses the actual vehicle on the day, so your certificate is what counts.
What normally causes it?
- A previous owner having the filter removed to avoid a repair bill
- A hollowed-out casing sold as a cheap alternative to a new filter
- A downpipe swap on a modified car that never had the filter re-fitted
- Buying a used car with the work already done and no disclosure
What a mechanic would check
The casing is inspected and the smoke test tells its own story. On a car I'm buying or inspecting I'd also look for fresh weld beads around the filter body and an ECU that has been mapped to ignore the missing sensors — the two usually go together.
What might need replacing
- A replacement DPF, genuine or a quality aftermarket unit
- The original mapping restored so the regeneration strategy works again
- Repair of whatever caused the original blockage, usually EGR, injectors or short-journey use
Can you still drive it?
The car drives, but it's failed the MOT and it's an offence to use a vehicle whose emissions equipment has been removed. There's no legal fix except putting a filter back on.
Repair cost
This is one of the dearer failures on the list, and cheap unbranded filters can block again quickly. Fix the underlying cause at the same time or you'll be buying two.
Before the retest
After exhaust work, start the car from cold and listen for blowing or ticking at the joints, and feel (carefully, before it gets hot) for any section that rattles or hangs low. Every original mounting rubber should be present. A sealed system matters for the emissions part of the test as well, because a leak upstream of a sensor can upset the readings.
Retest rules
If the vehicle stays at the test centre for repair and is retested within 10 working days, a partial retest is often free; some items qualify for a free retest if you bring the car back by the end of the next working day. The exact conditions are set out on GOV.UK, so confirm with the test centre before you leave. A major or dangerous failure means the car cannot legally be driven away unless the previous MOT is still valid and nothing dangerous was recorded — check your certificate first.
Related help
More exhaust failures · MOT failure decoder · Ask a mechanic about your certificate
This is general guidance from a UK mechanic, not a substitute for a physical inspection. Only the tester who inspected your vehicle can confirm what was recorded and why.