Many people dread the emissions part of the annual MOT test, passing this can be much easier if we know how the testing procedure works, and how we can overcome some of the obstacles which would make a vehicle fail the emissions component.

Diesel testing uses a probe inserted into the exhaust, this is simply a light with a collector, the light illuminates and the collector measures the amount of light as the exhaust pass between the two. This then gives us a reading as the electrical signal which is generated converts this into the reading. Diesel exhausts suffer from a couple of problems; these are the damp debris which collects inside the exhaust pipe, and the dry debris which collects inside the exhaust pipe.
Revving the engine is a vital component of testing as many people do not rev their engines fully, much of the accumulated debris does not come off during normal driving, during the emissions test, it does. Wet debris comes off as small lumps which pass through the collector and are read as such by reducing the light at the collector; dry debris comes off as coarse soot particles which also impede the passing light in the collector.

Now we know this we can act, but one other component comes into our equation, how good is our combustion and how much soot is it creating during normal driving? Basically it depends upon the condition of the engine, but can be improved.
Now we know we can improve our chances of passing the MOT emissions test by dealing with these factors.

One month before the MOT test I always advocate improving combustion, this is easily done by adding a cleaner specially designed for diesel injection systems, and many clean the exhaust deposits and reduce them. This takes time to work, this is why it is added one month prior to the test, it gives it time to clean and fully clean the injection system and remove the deposits, if it is used immediately prior to the test the deposits being removed will show up during testing.

One week before the test: service the vehicle fully, new air filters work more efficiently and ensure the best possible quality and quantity of air is reaching the engine. Replace the fuel filter, this ensures no water or other contaminants are in the fuel, and it is also working at its optimum. Replace the oil and oil filter, worn or used oil produces more fumes which pass through the engine as it is introduced back into the combustion chambers as part of the emissions control equipment fitted to the engine.

Book your MOT test at a dinnertime or afternoon slot, this gives you time to prepare; immediately prior to the test, adopt the following procedure:
Run the vehicle for at least 20 miles, make a proportion of this high speed running at motorway speeds if possible as this allows the engine to warm fully, this also reduces emissions, it also ensures the wet deposits are now dried. After about 10 miles rev the engine to its maximum revs on several occasions, this removes a considerable amount of the soot and accumulations in the exhaust, you will notice smoke which will gradually disappear with each successive high engine speed burst.

Drive your vehicle straight to the MOT testing station and insist the emissions are tested immediately while the engine is still hot and the deposits have been removed from the exhaust. This will ensure the best possible chance of passing the MOT emissions component.

If the vehicle fails the test there are more things you may do, if it is a marginal fail it may be that the system is measuring the deposits from the exhaust as they have not been removed effectively.
Remove the exhaust from the manifold, bung up the outlet at the rear of the vehicle, fill with water from the exhaust manifold end and allow to soak for several minutes, this will loosen a considerable amount of the stubborn deposits. These deposits form because most vehicles are not driven hard, during damp weather they stick to the exhaust like sludge or mud and become baked on, and this is due to the damp air entering the exhaust when the vehicle is cold. As the vehicle is started more deposits form over the top of the original deposits and they build up over time to form a considerable quantity of deposits.

Wetting the inside of the exhaust softens these accumulated deposits, remove the bung and allow the water to run, repeat this procedure several times and flush through with a hosepipe or Jet wash to remove as much as possible, until the exhaust internals are clean. Take considerable care not to let any water get into the turbocharger or the cylinder head as this can damage them; and if possible have the vehicle pointing slightly uphill.
Once the inside of the exhaust is fully cleaned it can be reconnected to the manifold, start the engine and be aware that a considerable quantity of dirty water will be expelled from the exhaust, this is water trapped in the silencers and it makes a mess.

Repeat the engine warming procedures prior to the emissions test to ensure the engine is fully warmed and submit for emissions testing again, it should have a significant drop in emissions and now pass.

One person I know usually has a problem with emissions failures, he now keeps a spare exhaust which is stainless steel, a group of them now fit this prior to the emissions test; in 6 years none of the vehicles have failed. This is removed and the original exhaust refitted after the emissions test, and the stainless one is fully cleaned and stored until one of the group have an MOT test.