DPF cleaning
Cleaning a clogged diesel particulate filter, and what the MOT’s published smoke limits mean for your car.
Monday to Saturday, 9am to 6pm. Sunday closed.
When to book it in
The DPF warning light is on.
Loss of power, or limp mode.
The cooling fan runs after you switch off, or fuel use has risen. Both can mean the filter is regenerating often.
Most of your journeys are short and slow.
Worth knowing before you book
A filter burns off the soot it collects during a long run at steady speed. Cars that only do short trips often never finish the job, and the filter clogs.
A warning light is not always a failed filter. A faulty sensor or pressure pipe can give the same light, so the cause needs confirming before anything is cleaned or replaced.
What the MOT does and does not cover
Diesel particulate filters are checked under the exhaust emissions section of the MOT, and the smoke limits are published as fixed numbers rather than left to a tester judgement.
DVSA MOT inspection manual
The limit is 0.7 m⁻¹ for a diesel first used on or after 1 January 2014, and 1.5 m⁻¹ for one first used between 1 July 2008 and 31 December 2013. Older engines carry higher figures, and a tolerance of 0.05 is added to the plated value before the vehicle is measured.
DVSA MOT inspection manual
On a diesel with a factory fitted particulate filter, visible smoke of any colour is recorded as a major defect. Evidence that a filter has been tampered with is a major defect as well.
DVSA MOT inspection manual
The manual also says what to do about a filter that has been cut open and rewelded. A tester should reject the vehicle unless there is evidence it was cut for a valid reason, and it gives filter cleaning as the example of such a reason.
DVSA MOT inspection manual
A missing emissions control system on a vehicle first used on or after 1 September 2002 is a reject, and the tester has no discretion over it.
DVSA MOT inspection manual
Where this shows up in the MOT test
The MOT inspects these areas. Each link goes to the section of the test that covers it.
Book it in, or ask us first
We are at 19 Dane Road, Bletchley, Milton Keynes MK1 1JQ.