The MOT emissions test is one of the parts of the annual test that many drivers know little about. You might understand that your car needs to produce emissions within certain limits, but what does the tester actually measure, and why do some vehicles fail when they seem to be running perfectly?
At PortsmouthTyre, MOT testing covers the emissions requirements applicable to your vehicle alongside the other checks required during the test. Understanding how the MOT emissions test works can help you spot potential problems early and avoid an unexpected failure.
What Is the MOT Emissions Test?
The emissions part of an MOT checks whether a vehicle’s exhaust emissions are within the permitted limits for its age and fuel type.
The exact test varies depending on the vehicle. Petrol and diesel cars are assessed differently because their engines produce different types and levels of emissions.
The tester may connect measuring equipment to the vehicle’s exhaust system and compare the results against the relevant limits. The test can also involve checking the vehicle’s emissions control equipment and dashboard warning lights.
The purpose is not to measure whether your car produces absolutely no pollution. Combustion engines inevitably produce exhaust gases. Instead, the test checks that emissions are within the standards applicable to the vehicle.
What Emissions Are Checked?
The gases and measurements involved depend on the type of engine and the vehicle’s age.
For petrol vehicles, the MOT can measure gases including:
- Carbon monoxide
- Hydrocarbons
- Carbon dioxide
- Oxygen
The test can also calculate the vehicle’s lambda value, which helps indicate whether the engine is operating with the correct air-to-fuel mixture.
Diesel vehicles are tested differently, with the opacity of the exhaust smoke measured under controlled conditions.
This means there is no single emissions limit that applies to every car. The relevant standard depends on factors including the vehicle’s fuel type, age and design.
Why Do Diesel Cars Have a Different Emissions Test?
Diesel engines operate differently from petrol engines and produce different exhaust emissions.
One of the most noticeable differences is particulate matter, which is why diesel vehicles use systems such as diesel particulate filters, commonly known as DPFs, to reduce particulate emissions.
During the MOT, the diesel emissions test measures smoke opacity. Excessive visible smoke can indicate an emissions problem and can result in a failure.
Diesel drivers should therefore pay attention to changes in exhaust smoke, particularly if the vehicle has started producing noticeably more smoke than usual.
Can a Petrol Car Fail Its MOT on Emissions?
Yes.
A petrol vehicle can fail its MOT if measured emissions exceed the permitted limits or if a relevant emissions-related component or warning system does not meet the required standard.
A vehicle might appear to drive normally while still producing excessive emissions. This is one reason why an emissions test is necessary rather than relying solely on how the engine feels.
Potential causes of high emissions can include faults with sensors, the fuel system, ignition components, catalytic converter or other engine-management equipment.
An illuminated engine management light can also be significant.
If the light remains illuminated because it indicates a relevant malfunction, the vehicle can fail its MOT even before considering the measured emissions.
What Causes High MOT Emissions?
There are many possible causes, ranging from relatively straightforward maintenance issues to more expensive component failures.
Common causes can include:
- Faulty oxygen sensors
- Problems with the fuel system
- Worn spark plugs
- Ignition faults
- Catalytic converter problems
- Air leaks
- Faulty engine sensors
- Exhaust leaks
- Diesel DPF problems
- Poor engine maintenance
The correct solution depends on the vehicle and the fault.
Replacing a component without first diagnosing the problem can be an expensive way of approaching an emissions issue. A diagnostic check can help identify where the problem is originating.
PortsmouthTyre offers engine diagnostic services in Portsmouth to help identify faults behind warning lights and engine-related problems.
What Is the Lambda Value?
Petrol drivers may come across the term “lambda” when discussing MOT emissions.
The lambda value provides an indication of the air-to-fuel mixture being used by the engine. Petrol engines generally need the correct balance of air and fuel for efficient combustion and effective operation of the emissions control system.
If the mixture is too rich or too lean, the vehicle may produce higher levels of certain emissions.
A faulty oxygen sensor can contribute to an incorrect mixture, although there are many other potential causes.
The lambda reading is therefore one of several measurements that can help identify whether a petrol engine’s combustion and emissions systems are operating correctly.
What Is a Diesel Smoke Test?
For diesel vehicles, the MOT emissions test involves measuring the opacity of exhaust smoke.
The vehicle is tested using a procedure designed to assess how much light is obscured by the exhaust emissions. Excessive smoke can indicate that the engine or emissions system is not operating correctly.
Diesel particulate filters have made modern diesel emissions systems more sophisticated, and a DPF problem can potentially lead to increased emissions and other warning signs.
If your diesel car regularly makes short journeys, it is particularly important to follow the manufacturer’s maintenance requirements and respond to DPF warnings promptly.
Can a DPF Problem Cause an MOT Failure?
It can.
A diesel particulate filter is designed to capture particulate matter from the exhaust. Over time, the filter can become loaded with soot and needs to regenerate to burn off the accumulated material.
A vehicle that is mainly used for short, low-speed journeys may have more difficulty completing the conditions required for regeneration.
A blocked or malfunctioning DPF can contribute to emissions problems and may cause warning lights to appear.
If your DPF warning light comes on, do not simply ignore it until the next MOT. Investigating the issue early can help prevent further problems and potentially more expensive repairs.
Does an Engine Management Light Mean an Automatic MOT Failure?
Not every dashboard warning light automatically means a vehicle will fail, but the engine management light can be significant during the MOT.
For applicable vehicles, the malfunction indicator lamp is checked as part of the test. If it remains illuminated because the system detects a relevant fault, the vehicle can receive a major defect.
This is why clearing the warning light immediately before an MOT is not a sensible solution.
If the light has appeared, have the underlying fault diagnosed. The problem could be relatively straightforward, but the warning exists because the vehicle has detected something that requires attention.
How Can You Reduce the Risk of an Emissions Failure?
There is no guaranteed way to make a vehicle pass its emissions test, but proper maintenance can reduce the risk of problems developing.
Regular servicing helps identify issues with the engine, exhaust and emissions systems before they become more serious.
You should also pay attention to changes in how the vehicle runs. Poor starting, rough idling, loss of power, unusual exhaust smoke, increased fuel consumption and warning lights can all be signs that something needs investigating.
If your MOT is approaching and you already suspect an emissions problem, having the car checked beforehand gives you time to diagnose and repair the fault.
For drivers looking after their vehicle between MOTs, understanding the difference between mini, interim, full and major car services can help you decide which level of routine maintenance is appropriate for your car.
Can You Prepare Your Car for an Emissions Test?
There are a few sensible things you can do before an MOT, although you should never attempt to manipulate the vehicle specifically to produce a temporary emissions result.
Make sure the vehicle is properly maintained and that the engine is operating normally. Check that there are no warning lights illuminated and investigate any unusual smoke, smells or engine behaviour.
Your car should also have sufficient fuel for the test and, where applicable, the correct levels of other fluids.
If an emissions warning has appeared recently, arrange a diagnostic check rather than hoping it will disappear before the MOT.
What Happens If Your Car Fails on Emissions?
If your vehicle exceeds the applicable emissions limits or has a relevant emissions-related defect, the MOT result will explain what has caused the failure.
The next step is to diagnose and repair the underlying problem.
There is no single repair for an emissions failure. A faulty sensor, catalytic converter, exhaust leak and diesel emissions problem require very different solutions.
A proper diagnosis can therefore save time and prevent unnecessary parts being replaced.
Once the required repairs have been completed, the vehicle can be presented for a retest in accordance with the applicable MOT retest arrangements.
Book Your MOT in Portsmouth
The MOT emissions test is designed to make sure your vehicle’s exhaust emissions remain within the standards applicable to its age, fuel type and design. Petrol vehicles are assessed using different measurements from diesel cars, while emissions control systems and relevant warning lights can also affect the result.
If your car is showing an engine management warning, producing unusual smoke or displaying other signs of an emissions problem, it is worth investigating before the MOT rather than waiting for a failure.
PortsmouthTyre provides MOT testing in Portsmouth at its Southsea workshop, alongside diagnostics, servicing, brakes and tyre services. Online booking is available, including same-day MOT appointments through the live booking system.
A little preventative maintenance can make a significant difference. Keeping your engine and emissions systems properly maintained not only improves your chances of passing the MOT but can also help your vehicle run efficiently and reliably throughout the year.
