MOT time can be a stressful time for many drivers across the UK. The fear is that the car could break down or fail due to another issue, and one of the more frequent reasons for cars failing the MOT in recent diesel cars is a blocked or dirty diesel particulate filter, or DPF for short.

Fortunately, DPF cleaning can actually help clear up a vehicle's MOT problems. For any diesel car owner, it is helpful to understand the job of the DPF, why it can become clogged and how it can be cleaned to make a difference.

What Is a DPF and What Does It Do?

A diesel particulate filter is a device installed in the exhaust system of most modern diesel-powered cars. Its job is to catch and hold the tiny soot particles that are produced when diesel fuel is burnt. Otherwise, these particles would be released into the air, which would lead to air pollution. From around 2009, the DPF was made mandatory for diesel vehicles sold in the UK, and it has a significant part to play in helping to maintain the vehicle's exhaust emissions within the legal limits set by the government.

The soot which builds up inside the DPF can increase over time. The filter does its own cleaning during normal driving through a process called regeneration, which involves burning the soot at high temperatures.

But when the car is driven on a lot of short trips, for example, short journeys around town or school runs, the engine never gets hot enough for this regeneration to occur correctly. This leads to a build-up of soot in the DPF, which prevents it from working properly.

How Does a Blocked DPF Affect an MOT?

MOT testing includes an emissions test. This test determines the amount of harmful emissions produced by the exhaust of a vehicle. The vehicle will probably produce more exhaust emissions than allowed if the DPF becomes blocked or damaged. This will mean that the emissions part of the MOT test will fail.

Starting from 2014, UK MOT testers have also been checking the DPF visually during the test. If the tester can see that the DPF has been removed or altered, they will automatically fail the MOT Northampton. In addition, if a vehicle is driving on public roads in the UK without the filter working, this is illegal as well, making regular filter maintenance even more important.

Another red flag that something is wrong is a warning light appearing on the dashboard regarding the DPF. Although having a warning light on the dashboard isn't an MOT failure, it's a hint that something is wrong, and it may be something serious before the MOT is carried out.

What Is DPF Cleaning and How Does It Work?

DPF cleaning is a professional service that removes the soot, ash and other deposits that build up in the filter. Garages and professionals in the UK use a number of techniques. A common way to do this is a chemical cleaning treatment, which uses a special cleaning fluid that's pumped into the filter to break down the deposits.

This can sometimes be done without removing the filter from the vehicle. The other approach is to simply remove the DPF and send it to a facility where it is cleaned using a high-pressure machine. Thermal cleaning is carried out in some specialist centres where the filter is heated under carefully managed conditions to allow the remaining ash to be burnt off.

The appropriate method will be dependent on the level of blockage and the type of vehicle. A professional tech will be able to determine the condition of the DPF and what type of DPF cleaning is best for the condition.

Can DPF Cleaning Actually Help a Vehicle Pass Its MOT?

The answer is yes, in many cases, the difference between a car passing or failing its MOT test lies with the condition of the DPF and whether it has been cleaned. 

A dirty DPF can cause high exhaust emissions, and it is possible to have the filter professionally cleaned and return the exhaust emissions to within allowed limits. A clean and effective DPF will significantly improve the chances of passing the MOT emissions test.

However, it is important to note that DPF cleaning Northampton is not always a complete solution. If the filter is damaged, not just blocked, for example, if the filter has broken or melted because it was too hot, then it will not be sufficient to clean the filter. 

In such situations, it may be necessary to replace the DPF altogether. A competent garage will not give the driver advice to have the DPF cleaned unless it is the right thing to do and the root cause has been identified; otherwise, the driver could be wasting their money on the service.

When Should a Driver Consider DPF Cleaning?

Whilst there are a number of signs that a DPF may need professional inspection prior to an MOT, there are also several indicators that can help determine whether it is time for professional cleaning. This may come in the form of a DPF warning light on the dashboard, reduced engine power (also known as reduced performance mode), higher fuel use or a definite smell from the exhaust.

If any of these symptoms are present, it is a good idea to book the car in for a DPF inspection well in advance of the planned MOT test date. Ignoring these warning signs can lead to an MOT failure, additional expense, and the hassle of another test.

By having the vehicle serviced regularly, motorists can help stop the DPF from becoming blocked. When a routine service is performed, a mechanic will have the opportunity to inspect the condition of the filter and advise whether some action needs to be taken.

Conclusion

It's a good service for people who own diesel cars and wish to give their car the best possible chance of passing its MOT. By keeping the diesel particulate filter in good condition, it will be easier to meet emissions standards, maintain engine performance, and avoid expensive repairs in the future. For all diesel car owners in the UK, keeping up with DPF maintenance is a straightforward way to avoid many problems on MOT day.