Pass the unfavourable emissions test at the MOT in just a few minutes and without spending a penny

One of the main reasons why diesel-engined cars can be rated unfavourably when they undergo the MOT is the emissions and opacity test. Many drivers who fail the MOT because of the results of this emissions test go to the workshop looking for a solution. Others turn to “anti-smoke” additives or directly opt for an engine decarbonisation process. And more and more people are resorting to home remedies that, on occasion, can cause serious engine damage.

Among the latter, it is increasingly common to hear of people setting off a firecracker in the exhaust pipe (I’m sure you’ve seen some video circulating on social media). There are also those who recommend adding a little petrol to the diesel. And I have recently been told about an increasingly widespread practice: applying high-pressure water (from a pressure washer or a Karcher-type machine) directly through the exhaust to “clean” the particulate filter.

IMPORTANT: do not even think of resorting to any of the solutions mentioned above Not only are they ineffective, but they can cause very serious and expensive problems to the exhaust system components or to the injection system.

If there is no fault or misadjustment in the engine, in most cases, as we are going to see, the solution to turn an unfavourable emissions test on a diesel into a favourable one at the MOT is much easier, and it is within everyone’s reach.

What is the opacity test for diesels carried out at the MOT?

The regulations for measuring opacity at the MOT have changed several times in recent times. This measurement records the level of smoke from the exhaust, to check that a maximum established limit is not exceeded, which varies depending on the age of the car. Put another way: what it quantifies is how black (opaque, hence the word opacity) the smoke coming out of the exhaust is. High opacity levels are a sign of incomplete combustion in the engine, meaning the diesel is not burning correctly.

The test is carried out by taking the engine up to the injection cut-off while accelerating in neutral. All auxiliary energy-consuming systems – lights, radio, air conditioning, etc. – must be switched off during the test. Normally three measurements are taken, and the most favourable one is kept.

Be very careful, because unlike petrol-engined cars, the maximum pollution limit applied at the MOT emissions test is the one set by the manufacturer when approving the engine, which appears on the so-called “emissions sticker” or “opacity sticker”. You can usually find it on the door frame, on the inside of the bonnet or on a front suspension turret. If the vehicle does not have an emissions sticker, then the values in the MOT Procedure Manual are taken as reference.

The limits are as follows:

• Euro 6 diesel cars: maximum opacity 0.7
• Euro 4 and Euro 5 diesel cars: maximum opacity 1.5
• Naturally aspirated diesel cars first registered before 1 July 2008: 2.5
• Vehicles built before 1 January 1980: exempt from the test

Can I keep driving if the emissions test is unfavourable?

It is important to bear in mind that the opacity test measures the black smoke the car emits, but your diesel may fail it even if you cannot visually see it emitting much smoke from the exhaust.

The test does not check whether the EGR valve or the particulate filter are working correctly; it may happen – although it is unlikely – that a car with these components in poor condition passes the test… and it is quite common for the opposite to occur, that is, for a diesel car to fail the emissions test without having any fault.

In any case, the message that appears on the report the MOT will give you if you fail the emissions test is a major defect, with the added issue that the car may only be driven to the workshop to fix the fault.

How to turn a failed emissions test into a pass

One last consideration before moving on to a practical case: the first thing you should do when it is time to take your diesel car for its MOT is make sure that the engine oil level is correct and that the engine has reached its optimal operating temperature. Ideally, you should have driven at least around 20 km before taking the test.

If, even so, the emissions test result is unfavourable, before taking any other measure I recommend doing the following. Leave the MOT station and head for a motorway, if possible. Wherever the speed limit of the road allows it, until you join the motorway, do not be afraid to accelerate the car up to the highest part of the rev counter in low gears (second or third). And once on the motorway or dual carriageway, drive for about 25 km, forgoing fifth or sixth gear (if you have one), so that the engine spends a good part of the journey turning around 3,500 rpm, occasionally reaching 4,000 rpm.

By doing this, the excess carbon build-up that may have accumulated in the exhaust system will be removed, which builds up in large quantities in diesel cars that make short journeys where the engine never reaches its optimal temperature. Or that are often driven around town or by abusing long gears, a style of driving that greatly favours the formation of carbon build-up, not to mention the saturation of the particulate filter.

Once this is done, go back to the MOT station, you can take the emissions test again without having to go through the whole process… and most likely the result will change to favourable. If not, then you can resort to a fuel additive, although things are unlikely to improve, and then you will have to go to the workshop to check, first and foremost, the air filter and the exhaust system itself, or move on to check the injection or the rest of the anti-pollution systems (EGR, particulate filter, etc.).

A practical case: from unfavourable to favourable in a matter of minutes

As a real example, I took a car we have used at home for years to the MOT, a Ford Ka 1.3 TDCi from 2009 equipped with a particulate filter. The car had just had its annual service at the usual workshop and had never had any problem at the MOT before. The opacity values in the last inspections (I make a habit of keeping at least the last 2) were below 0.4.

It should be noted that, in this case, the opacity limit is the one recorded on the aforementioned opacity label, which you can see above, where the maximum value cannot exceed 1.12.

However, for the first time, the result of the test was unfavourable, with an opacity value of 1.21, the best of the 8 measurements taken, which always oscillated between 1.6 and 1.3. And it was no coincidence: due to various circumstances, over the last month the car had been used precisely for very short journeys, without going out on the open road.

We carried out the process mentioned above, in this case a 35 km round trip between home and the MOT station, giving the engine a good workout. And on the first measurement after returning to the MOT, just a few minutes after the first one, it passed, with a value of 0.9, and without needing to go to the workshop or resort to “black magic tricks” to get rid of the bad smoke. Sticker on the windscreen…. and see you next year.

Related information of interest

• The fines you can get for having your car on the road without a valid MOT
• The changes coming into force at the MOT in June
• Expired MOT: the penalties you can receive
• Engine fault warning light: if it is on you will not pass the MOT