During the diesel smoke test the opacity reading was above the limit for your vehicle. In plain terms, too much soot is coming out under a free acceleration test.
Wording as it commonly appears on an MOT certificate: “Smoke density exceeds the limit for this vehicle”.
What does it mean?
During the diesel smoke test the opacity reading was above the limit for your vehicle. In plain terms, too much soot is coming out under a free acceleration test.
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 partially blocked or ash-loaded DPF
- EGR valve stuck open or a clogged EGR cooler
- Injector wear affecting spray pattern
- A car that lives on short cold journeys and never gets hot enough to regenerate properly
- A stretched service interval leaving old oil and a clogged air filter
What a mechanic would check
Read the DPF data — soot load, ash load and the time or distance since the last successful regeneration. That data tells you whether the filter is genuinely full or just never given a chance. I'd also look at EGR position and get the car properly hot on a road test before drawing any conclusions.
What might need replacing
- A good long run and a forced regeneration
- EGR valve cleaning or replacement
- DPF cleaning, or replacement where ash-loaded beyond recovery
- A service with the correct low-ash oil for a DPF car
Can you still drive it?
It's a failure. A car smoking heavily is usually telling you something is wrong further upstream, so don't just retest and hope — the underlying cause tends to get worse and DPF work gets more expensive the longer it's left.
Repair cost
Ranges from a service and a decent run to DPF replacement, which is one of the more expensive jobs on a modern diesel. Get the DPF data read before anybody quotes you a filter.
Before the retest
Fix the cause, not the reading: a fault code, sensor or air leak that made the car fail will make it fail again. Make sure no engine warning light is lit. Drive the car properly on the way to the retest — at least 20 minutes at normal road speeds — so the engine and catalytic converter are fully up to temperature. Never remove or tamper with a diesel particulate filter; that is an automatic failure.
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 emissions 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.