05/07/2011
As a diesel vehicle owner in the UK, you might be wondering about the MOT emissions test and how recent changes might affect your car. The MOT test is a crucial annual check to ensure your vehicle is safe and roadworthy, and emissions testing plays a significant role in this. With new regulations introduced in May 2018, many vehicles, particularly diesels, have been failing due to stricter emissions standards. This guide will delve into the specifics of MOT emissions testing for diesel vehicles, why these changes were implemented, and what you can do to prepare your car for its MOT to maximise its chances of passing.
Why are MOTs Including Emissions Tests?
The inclusion of emissions testing in the MOT is a direct response to growing environmental concerns and the established link between air pollution and vehicle exhaust fumes. The government's commitment to reducing carbon emissions and improving air quality means that vehicles must meet certain standards to be deemed roadworthy. The MOT test, therefore, assesses not only the mechanical safety of a vehicle but also its environmental impact. The test involves inserting a probe into the exhaust tailpipe while the engine is running to measure the volume of harmful gases and hydrocarbons. If these levels exceed the prescribed limits, or if the calculated 'Lambda' value is outside the acceptable range, the vehicle will fail.
The Impact of New MOT Emissions Rules on Diesel Cars
The MOT emissions regulations underwent a significant overhaul in May 2018 as part of the EU Roadworthiness Package. These changes introduced more stringent rules and new defect categories, with a particular focus on diesel engines. Diesel vehicles are now subject to greater scrutiny regarding their exhaust emissions, the presence of visible smoke, the engine management warning light, and the condition of the diesel particulate filter (DPF). The DPF is a critical component designed to capture soot and reduce particulate emissions. Any tampering with or removal of the DPF is now a direct cause for failure. Furthermore, an illuminated engine management light, which often indicates a problem with the emissions control system or DPF, will also result in a failed MOT.
It's not just diesel cars that have been affected. Petrol cars now also face a 'major' fault and an automatic fail if they emit oily or blue smoke for more than five seconds. These stricter rules mean that issues affecting a car's emissions are now commonly classified as 'major' or 'dangerous' faults, leading to a fail.
Understanding the MOT Defect Categories
The MOT test categorises defects to clearly define the severity of an issue and its implications for vehicle safety and the environment. Understanding these categories is key:
| Defect Category | Defect Description | Test Result |
|---|---|---|
| Minor | No significant effect on the safety of the vehicle or impact on the environment. | Pass – Fault should be repaired as soon as possible. |
| Major | May affect the vehicle’s safety, put other road users at risk, or have an impact on the environment. Most emissions issues fall into this category. | Fail – Repair immediately. The car can be driven home, but should only be used again for driving to the rescheduled test. |
| Dangerous | Direct and immediate risk to road safety or poses a serious impact on the environment. | Fail – The car cannot be driven until it has been repaired. |
As you can see, most emissions-related issues are now classified as 'Major' faults, meaning your vehicle will fail its MOT if it doesn't meet the required emissions standards.
Key MOT Emissions Rules for Diesel Vehicles
To ensure your diesel vehicle has the best chance of passing its MOT, be aware of these specific rules:
- No Visible Smoke: Your vehicle must not produce visible smoke from the exhaust. This includes any colour of smoke, not just black.
- Emissions Limits: Your vehicle must not exceed the specified emissions restrictions for diesel engines.
- Diesel Particulate Filter (DPF): The DPF must be present and must not have been tampered with or removed. A missing or modified DPF is a definitive reason for failure.
- Engine Management Light: The engine management warning light must not be illuminated. This light often indicates a fault within the emissions control system or the DPF.
How to Reduce Your Diesel Car's Emissions for an MOT
Failing an MOT can be inconvenient and costly. Taking proactive steps to prepare your vehicle can significantly improve your chances of a first-time pass. Here’s how you can help reduce your diesel car’s emissions:
1. Regular Servicing and Maintenance
Many emissions issues stem from poor engine health and a lack of regular maintenance. An annual service is essential for ensuring your engine runs as efficiently and cleanly as possible. During a service, a qualified mechanic can identify and rectify potential problems before they escalate and affect your emissions readings. Issues like old engine oil, clogged air filters, and worn spark plugs can all contribute to increased exhaust fume toxicity.
2. Address Underlying Issues Promptly
If you're aware of any problems with your car that might be affecting its performance or fuel efficiency, it's best to get them repaired before your MOT. Common culprits for high emissions include:
- Blocked Air Filters: Restrict airflow to the engine, leading to inefficient combustion.
- Faulty Exhaust Systems: Leaks or damage can affect emissions readings.
- Old Engine Oil: Degraded oil can lead to increased friction and less efficient engine operation.
- Spark Plug Problems: In petrol engines, but can affect overall engine health.
- Defective Oxygen Sensors: Crucial for regulating the air-fuel mixture; a faulty sensor can lead to increased emissions.
Symptoms like black exhaust smoke, a noticeable drop in fuel economy, or reduced power output are often indicators of these underlying issues. Getting them diagnosed and fixed before your MOT is a wise move.
3. Clean Your Fuel System with Additives
Keeping your fuel system clean and running efficiently is vital, especially when preparing for an MOT. Fuel additives, like those offered by Redex, can provide a deep clean to your fuel system, helping to lower emissions and potentially improve your car's MOT pass rate.
Using a Pre-MOT Emissions Reducer:
Products like Redex Emissions Reducer are specifically designed to be used in the run-up to an MOT. When added to your fuel tank, these additives circulate through the fuel system and engine, performing several key functions:
- Reduce Exhaust Emissions: By cleaning deposits and improving combustion, they help lower the levels of harmful gases.
- Restore Engine Performance: A cleaner fuel system leads to more efficient combustion, restoring lost power and responsiveness.
- Improve Engine Life: Removing harmful deposits can help protect vital engine components.
- Aid MOT Pass: By reducing emissions, they significantly increase the likelihood of passing the emissions test.
The application is simple: just pour the contents of the bottle into your fuel filler cap. The additive then works its magic as you drive, cleaning injectors and other components.
Long-Term Fuel System Cleaning:
For ongoing maintenance and to keep emissions consistently low, consider using Redex System Cleaners as a regular part of your car's upkeep. These are available for both petrol and diesel vehicles and offer:
Redex Diesel System Cleaner Benefits:
- Quickly cleans and clears particles from your diesel fuel system.
- Advanced formula designed for diesel engines.
- Provides a complete system clean.
- Helps your car pass its MOT with long-term use.
By regularly using a reputable fuel system cleaner, you can maintain optimal engine performance and ensure your diesel vehicle is always in the best possible condition to meet emissions standards.
Frequently Asked Questions (FAQs)
Q1: What happens if my diesel car fails the MOT emissions test?
If your diesel car fails the emissions test, it will receive a 'Major' or 'Dangerous' fault classification and thus fail the MOT. You will be issued with a refusal of an MOT test certificate. You cannot legally drive your vehicle on the road until the necessary repairs have been made and it has passed a re-test. You can only drive the car to a pre-booked MOT re-test or to a place of repair.
Q2: Can I use a fuel additive to pass my MOT?
Using a fuel additive, such as a pre-MOT emissions reducer, can help to clean your fuel system and improve combustion, which in turn can lower your emissions. While it's not a guaranteed 'magic bullet' for every failing vehicle, it can significantly improve your chances of passing, especially if the underlying cause of high emissions is a dirty fuel system. It's best used as part of a broader maintenance strategy.
Q3: What is Lambda in an MOT emissions test?
Lambda is a measure of the air-fuel ratio in your engine. The MOT test checks if the Lambda value is within the correct range for your specific vehicle type. An incorrect Lambda value can indicate that the engine is not burning fuel efficiently, leading to higher emissions.
Q4: Is the DPF mandatory for my diesel car's MOT?
Yes, the Diesel Particulate Filter (DPF) is a mandatory component for diesel vehicles that are fitted with one from the factory. The MOT test will check if the DPF is present and has not been tampered with or removed. If the DPF has been illegally removed or modified, your vehicle will fail its MOT.
Look out for warning signs such as the engine management light illuminating on your dashboard, a noticeable increase in black smoke from the exhaust, a decrease in fuel economy, or a loss of engine power. If you notice any of these, it's advisable to have your vehicle inspected by a mechanic before booking your MOT.
By staying informed about the MOT emissions regulations and taking proactive steps to maintain your diesel vehicle, you can significantly increase your chances of a successful MOT test. Regular servicing, addressing any mechanical issues promptly, and considering fuel system cleaners are all effective strategies to ensure your car remains compliant and environmentally sound.
If you want to read more articles similar to Diesel MOT Emissions: What You Need to Know, you can visit the MOT category.