New soot test for the MOT: everything you need to know about the particle test

From 1 January 2023, a new MOT soot test applies in the Netherlands for all diesel engines fitted with a factory-installed diesel particulate filter. This is also known as the “particle counter test”. The focus of this test is on diesel engines where the DPF has been removed or is not functioning properly. A diesel car with excessive particle emissions will immediately fail the inspection.

In many cases, this means the DPF has to be replaced, which can lead to a significant repair cost. However, there are also possible causes that are not directly related to the DPF itself. More on that later.

In Belgium, this test was already introduced on 1 July 2022. It was initially also planned for the Netherlands at that time, but due to delivery problems with the testing equipment, the introduction was postponed until 1 January 2023.

What is a particle counter and how does it work?

During an MOT inspection, a car is checked on several points. Since 1 January 2023, this also includes the fine particles emitted by diesel cars. A particle counter measures the number of particles per cm³ while the engine is idling.

The test takes approximately 90 seconds, and it is important that the engine is properly warmed up. A hose-like device is connected to the car’s exhaust, after which the MOT test can be performed. The particle counter can detect nanoparticles and determine whether the DPF is functioning properly.

There are several devices available on the market, using two different measurement techniques: condensation particle counting (CPC) and diffusion charging (DC). We briefly explain both below.

CPC technique

With the CPC technique, soot particles are measured using a laser. Because the particles are normally too small to measure, they are enlarged using a specific type of alcohol. This allows each particle to be measured individually by the laser, resulting in an accurate measurement. The maximum deviation is around 10%.

For garages, this technique is often recommended because it is a more durable way of measuring. The contamination does not remain in the measuring chamber because alcohol is used. Equipment using this technique is usually more expensive than equipment using the DC technique.

DC technique

The DC technique uses ionisation. The particles are electrically charged, and this charge is then measured. Based on this, the device calculates how many particles pass through.

This measurement method has a maximum deviation of 25%. Besides being less accurate, it is also more sensitive to contamination, which can result in a shorter service life. The advantage is that these devices are generally much cheaper.

Why is a particle test performed?

Soot particles, also known as fine particulate matter, are harmful to both people and the environment. To meet climate targets and reduce nitrogen and particulate matter emissions, authorities have decided to enforce vehicle emissions more strictly. The question is: do vehicles meet the environmental requirements?

Emission regulations are intended to reduce particle emissions. Recent studies have shown that current measuring instruments are not sensitive enough to detect these particles properly. As a result, they do not adequately check whether a vehicle has a defective or missing DPF. That is why a particle test using suitable equipment is now being introduced during the MOT inspection.

At the time of writing, Belgium is the only country where a particle test is already being carried out. From 1 January 2023, several EU countries will follow, making the particle test mandatory in countries such as Germany, Switzerland and the Netherlands. More EU countries are expected to follow in the future.

As you can see, Germany and Switzerland are even stricter, which means a car can fail the inspection sooner. We have now covered what a particle counter is and why the test was introduced. Next, we will look at how the particle counter works, what you can do if your result exceeds the limit, and what you can do preventively to avoid future problems as much as possible.

Possible causes of inspection failure and how to recognise them

During an MOT inspection, a car is checked on several points, including the particles emitted by a diesel car. A particle counter measures the number of particles per cm³ while the engine is idling. The test takes approximately 90 seconds, and it is important that the engine is properly warmed up. A hose-like device is attached to the car’s exhaust, after which the MOT test can be performed.

First of all, it is important to know that there may be several reasons why the limit is exceeded. These include:

• The DPF is damaged and contains small cracks
• The DPF is regenerating
• The EGR valve remains open
• Excessive soot formation due to dirty injectors or similar causes

Unfortunately, it is often difficult to identify the exact cause of the high particle count. However, you can use certain tips to increase your chances of passing the inspection. These tips are explained below.

The DPF is damaged

If a DPF is damaged, it can no longer filter optimally. Unfortunately, in this case, the only real solution is to have a new DPF fitted. You could also choose a used DPF that still functions properly, but this often involves additional risks.

Tip: If a lot of soot is visible inside the exhaust and you can wipe it off with your finger, this is often a sign that the DPF is showing signs of wear and may be damaged. This is not a guarantee, but it can be an indication.

DPF regeneration

When a DPF is relatively full, it will start to regenerate. Regeneration shortly before a particle test has a negative effect on the result. We therefore recommend not going to the inspection immediately after the regeneration process. Wait a few hundred kilometres before having your car inspected.

Tip: You may notice the regeneration process because the idle speed temporarily increases, the engine sometimes shakes slightly, or the engine sound changes.

The EGR valve remains open

If an EGR valve remains open, for example because it has become heavily contaminated, this has a negative effect on the particle test result. Exhaust gases are then partly routed via the EGR and the intake back into the combustion chamber. This means less clean air and oxygen are available, resulting in more soot formation.

An important point, especially with newer engines, is that the EGR valve is open by default so that warm exhaust gases help the engine reach operating temperature more quickly. By letting the car idle for around 20 minutes, the EGR valve in these newer engines will normally close because the engine has warmed up. If the EGR valve is stuck due to carbon buildup, it is better to clean the EGR valve.

Tip: An EGR valve that remains open can often be recognised by an illuminated warning light, loss of power, increased fuel consumption, black exhaust smoke, irregular idling and poor starting.

Excessive soot formation

Besides the issues above, any problem that causes increased soot emissions can negatively affect the particle test. Examples include:

• Dirty injectors, resulting in poor injection and poor combustion
• Dirty intake valves, resulting in less oxygen and poor combustion

Tip: Poor combustion can often be recognised by reduced power, higher fuel consumption and starting problems. Oil consumption is recognisable because you need to top up oil regularly, and it may also be visible through blue exhaust smoke.

What can you do yourself to pass the inspection?

To begin with, the new soot test for the inspection is not entirely without controversy. There has been criticism because the same car can produce varying results. However, based on this blog, many of these varying results can hopefully be explained more clearly.

With this information, you can also create “optimal conditions” to increase your chances of passing the inspection. We have listed several tips below.

Tip 1: Let the car idle for around 20 minutes

As mentioned earlier in this blog, especially with newer engines, the EGR valve may still be open when you have just started the car. Exhaust gases help warm up the engine.

By letting the car idle for around 20 minutes before the inspection, the EGR valve will generally close by itself because the engine has reached operating temperature. This can significantly improve the test result.

Tip 2: Do not let the DPF regenerate

It may sound strange for a “new soot measurement”, but it is true: the more filled the DPF is, the better the result can be for the particle test. If a DPF is slightly damaged, you may experience fewer negative effects if the filter is well filled with soot.

The advice is therefore to make sure the DPF does not regenerate shortly before the inspection.

For the overall performance of the car, separate from the particle test, it is still advisable to clean the DPF. You can read more about that in this blog. Useful for after the inspection.

Tip 3: Clean the injectors

Dirty injectors can be the cause of a high particle reading. When an injector is dirty, the oxygen-fuel ratio in the combustion chamber is no longer optimal, resulting in soot formation.

To rule this out, it is advisable to clean the injectors with an additive. Lindemann Smokeless is one example.

In this case, we recommend adding the product to a full tank. Preferably add it before refuelling so it mixes properly with the fuel. After driving through half a tank, the injectors are usually cleaned with this product.