Your Guide to Diesel Smoke Tests & UK MOT

Understanding your vehicle's exhaust emissions is not just about environmental responsibility; it's also crucial for ensuring your car complies with UK roadworthiness standards. Diesel vehicles, in particular, are subject to stringent regulations regarding the smoke they emit, especially during the annual MOT test and roadside enforcement checks. This comprehensive guide will delve into what constitutes acceptable diesel smoke, explain the intricacies of the metered diesel smoke test, and clarify the significant changes introduced to the MOT rules in May 2018.

What is a Metered Diesel Smoke Test?

When a vehicle burns fuel, gases are inevitably emitted from its exhaust pipe. Sometimes, visible smoke also accompanies these emissions. While new car models undergo rigorous testing to measure these emissions, existing vehicles are regularly checked during their annual MOT test and through roadside enforcement. A metered diesel smoke test is a specific procedure designed to quantify the opacity of exhaust smoke, ensuring it falls within legal limits. It's a critical component of vehicle compliance, particularly for diesel engines, which historically have been more prone to visible smoke.

Vehicle Smoke Emissions Standards

The standards for smoke emissions vary significantly depending on the age and type of your diesel vehicle:

Diesel Vehicles Manufactured Since 2013

For diesel vehicles produced from 2013 onwards, the expectation is that there should be virtually no visible exhaust smoke emissions. The only exception is small quantities of water vapour, which might resemble steam from a boiling kettle under certain atmospheric conditions. A clear indicator that your emission control system is functioning correctly is a clean exhaust tailpipe, devoid of any soot on the inside. Conversely, a sooty tailpipe can signal an issue, most commonly with the Diesel Particulate Filter (DPF). The DPF is designed to trap soot and ash, drastically reducing harmful emissions. Should a fault arise, a warning lamp on your vehicle’s dashboard will often illuminate, advising you of a potential problem. It's important to note that a clean tailpipe strongly suggests your DPF is performing as it should, whereas a sooty one, unless recently repaired, indicates a likely DPF malfunction.

Diesel Vehicles Manufactured Before 2013

Older diesel vehicles are permitted to emit small quantities of smoke. However, this smoke should never be significant enough to obscure the visibility of other road users. You might notice smoke more when accelerating, but it should not be so pronounced that it catches your attention when checking your mirrors, nor should it cause soot to accumulate on the rear of your vehicle. If you observe such conditions, it's a strong indication that your vehicle requires inspection and potentially repair, as it suggests an issue beyond normal operating parameters for older diesels.

Historic (Classic) Vehicles

Historic vehicles, often referred to as classic cars, are allowed to emit larger quantities of visible smoke compared to modern vehicles. Nevertheless, even for these vehicles, the fundamental rule remains: the smoke must never obscure the vision of other road users. This ensures that while appreciating vintage machinery, road safety is not compromised.

Enforcement Checks and Penalties

As a vehicle owner, you bear the responsibility for ensuring your vehicle is in a roadworthy condition for use on public roads. Enforcement officers have the authority to conduct roadside emission spot checks. If your vehicle is observed emitting 'avoidable smoke', it may be subjected to a metered diesel smoke test. Non-compliant vehicles can face significant repercussions:

• Notice of Defects: The driver or owner may receive a notice requiring the identified defects to be rectified.
• Prohibition Notice: In more severe cases, a prohibition notice might be issued, preventing the vehicle from being driven further until the defect has been properly rectified.
• Fixed Penalties: A driver of a non-compliant vehicle may be offered a £30 fixed penalty.
• Court Fines: If prosecuted, a court can impose a fine of up to £1000 for standard vehicles, or up to £2,500 for Goods Vehicles and Buses.
• Additional Penalties: Further penalties may apply if other defects are detected, such as tampering with the emission control system.
• Danger to Road Users: If a vehicle poses a danger of injury to other road users due to its emissions, this could result in a £60 fixed penalty ticket and three penalty points. Prosecution could lead to a court fine of up to £2,500 (£5,000 for Goods Vehicles and Buses), alongside a prohibition on further use of the vehicle.

The 2018 MOT Changes: A Deep Dive into Diesel Smoke Rules

The introduction of new MOT Rules in May 2018 brought about considerable confusion, particularly concerning diesel vehicles and the smoke test. These changes impact a significant number of diesel cars, both those fitted with a DPF and those without. Understanding the specific rules for each category is vital for passing your annual inspection.

DPF-Equipped Cars

For cars equipped with a DPF, the rules are very strict: if the inspector observes any visible smoke of any colour during idle or the actual MOT test, it will result in an immediate fail. The diesel smoke test for DPF-equipped cars is now conducted against the manufacturer’s plate value. This figure is typically printed on the VIN plate or a sticker often found on the inner door pillar or sill. While most DPF cars came with these figures, default values are used if the figure is not shown or if the plate has been removed. The default values depend on the car's age:

• Diesel Cars manufactured after 2014: 0.7m-1
• Diesel Cars manufactured after July 2008: 1.5m-1

It is crucial to remember that these default values only apply if the MOT inspector cannot locate the plate value on your vehicle. If the plate value is found, it overrides the default figures, becoming the maximum acceptable value for the test. Should your vehicle’s smoke emissions exceed this plate value, it will fail its MOT, even if the measured emissions are still technically below the default values for its age.

Non-DPF Equipped Cars

For diesel cars not fitted with a DPF from the factory, the visible smoke rule is less stringent. Your car is still permitted to emit small amounts of smoke of any colour at idle or during the test. However, an inspector can still fail the car on a visual test if they deem the vehicle to be emitting excessive levels of blue or black smoke. So, if your car produces dense smoke akin to an old diesel bus, it could still fail. Small amounts of smoke, however, will not trigger a fail, unlike DPF-equipped vehicles.

Prior to May 2018, all older diesel cars were tested to a default value of 3.0m-1, which often allowed even high-mileage diesels with minor issues to pass. The new rules, however, significantly tighten these limits by testing the car in line with its original manufacturer’s emissions test figures. Similar to DPF cars, if your car has an emissions figure printed on an engine plate or sticker (e.g., in the boot or on a door frame), that figure will be used as the maximum limit. If no figure is printed or found, default figures based on the vehicle’s age apply:

• Diesel Cars Manufactured After July 2008: 1.5m-1
• Diesel Cars Manufactured Before July 2008 (Non-Turbo): 2.5m-1
• Diesel Cars Manufactured Before July 2008 (Turbo Diesel): 3.0m-1

Therefore, if your car was manufactured before July 2008 and lacks a manufacturer’s emissions figure, it will be tested to the original default figures of 2.5m-1 or 3.0m-1. However, if a plated figure is available, that becomes the maximum smoke value, and exceeding it will result in an MOT failure, even if the emission level is below the age-based default.

Finding Your Vehicle's Emission Plate Value

The smoke output figures are typically presented within a square box on either the engine bay plate or a sticker elsewhere on the car, commonly on a door frame or in the boot. For instance, the figure might be stamped onto the VIN plate located under the bonnet at the front of the engine bay. An example cited previously involved a 2005 vehicle, previously tested to 3.0m-1, now subject to a much tighter 1.1m-1 limit due to its plate value. Another example showed a figure of 1.4m-1 on a sticker on the inner driver's door frame for a vehicle manufactured before July 2008, which would have previously been tested to 3.0m-1. These examples highlight that manufacturer data doesn't follow a simple pattern; a newer car might have a higher plated limit than an older one.

Critique of the New Rules

The implementation of these new MOT changes has drawn criticism for several reasons. There's no clear pattern to how manufacturers applied these test figures, nor is there an explanation of the conditions under which the original tests were conducted. Crucially, no tolerance is added for the natural loss of efficiency that all cars experience with age and mileage. Expecting a 14-year-old car with 250,000 miles to perform as efficiently as a prototype on a controlled test track is seen as unrealistic. Furthermore, concerns have been raised about the authenticity of some plate figures, especially in the wake of scandals like "Diesel Gate," where figures were manipulated. This raises the possibility that some cars are now being tested to levels that were perhaps unattainable even when new, leading to a significant increase in MOT failures for diesel cars since May 2018.

Given the perceived unfairness and variability, some owners explore ways to have their vehicles tested under the original default figures. If the figures are on a sticker or label on a door frame or boot, they are relatively easy to remove, and labels can go missing or get damaged. If the data is on the engine bay plate or anything containing the VIN number, it's more problematic, as the VIN must be present for the MOT. However, companies exist that can replicate VIN plates with the emissions box left blank, a practice that is not currently illegal if the vehicle is yours and the VIN is original. While this article simply relays information circulating, it highlights the challenges and frustrations many owners face.

Beyond Smoke: Other Critical MOT Changes for Diesel Vehicles

In addition to the specific smoke test modifications, the May 2018 MOT changes introduced other significant checks that affect both DPF and non-DPF equipped diesel vehicles:

• DPF Tampering or Missing: If your car was factory-fitted with a DPF but it is now missing or shows obvious signs of tampering (e.g., recent welding, removed sensors), it will fail the MOT. You may be asked to provide evidence of genuine work, such as professional DPF cleaning.
• EGR Valve Issues: If the Exhaust Gas Recirculation (EGR) valve is visually modified, removed, or obviously blanked off, this will result in a fail.
• Leaking of Fuel, Oil, or Other Fluids: Your car will fail if it's leaking fluid to an extent that poses an environmental threat. Guidelines state a fail is given if a pool wider than 75mm (7.5cm) forms in 5 minutes. Minor leaks will continue to be advisories.
• Engine Management Light (MIL / Check Engine) Illuminated: A major change is that if your Engine Management Light (MIL) is illuminated at the time of the test, it will now result in a fail. There's also an initial test to ensure the MIL illuminates when the ignition is switched on and then extinguishes, preventing the light from being disabled.

Common Causes of Engine Management Light Illumination (and DPF Issues)

An illuminated MIL should always be investigated promptly, not left until just before the MOT. While many faults are related to sensors or wiring, some indicate deeper problems that can prevent the DPF from regenerating, potentially leading to costly clogging. Here are some common culprits:

• Glowplug Failures: One or more faulty glowplugs, or a failure in the glowplug fuse, relay, or controller, can impact combustion efficiency, leading to MIL illumination.
• DPF Pressure Sensor Failure: Malfunctioning pressure sensors on either side of the DPF filter prevent the system from accurately monitoring soot levels, thus inhibiting regeneration cycles.
• Exhaust Gas Temperature Sensor Failure: Incorrect temperature readings can disrupt the DPF regeneration process, as regeneration requires specific exhaust temperatures.
• EGR Blocked or Faulty: A blocked or faulty EGR valve affects exhaust gas flow and engine performance, often triggering the MIL and impacting DPF function.
• Coolant Sensor Faulty: An incorrect coolant temperature reading can prevent the engine from reaching optimal operating temperature for DPF regeneration.
• MAF Sensor Faults: A faulty Mass Air Flow (MAF) sensor can provide inaccurate air intake readings, affecting fuel-air mixture and combustion, which in turn impacts emissions and DPF operation.

Any of these issues can cause the MIL to illuminate and, critically, prevent your DPF from regenerating. This can lead to the DPF becoming clogged, necessitating expensive cleaning or even replacement. It is highly advisable to use a code reader to investigate MIL errors at your earliest convenience and rectify any faults quickly, especially those listed above, as the DPF will not regenerate while these issues persist.

Frequently Asked Questions About Diesel Smoke and MOTs

Can a DPF test fail a car?

Yes, absolutely. If a DPF-equipped car emits any visible smoke of any colour during the MOT test or at idle, it will fail. Furthermore, if the DPF is found to be missing or tampered with, it will also result in a fail.

What are the smoke emission standards for newer diesel vehicles?

For diesel vehicles manufactured since 2013, there should be no visible exhaust smoke other than small quantities of water vapour. A clean exhaust tailpipe is a good indicator of proper emission control system function.

What if my exhaust tailpipe is sooty?

A sooty exhaust tailpipe, especially on a vehicle made since 2013, often indicates a problem with the Diesel Particulate Filter (DPF). It suggests that the DPF is not effectively trapping soot and ash, and the vehicle may require inspection.

Are older diesel vehicles allowed to emit smoke?

Older diesel vehicles (pre-2013) may emit small quantities of smoke, particularly when accelerating. However, this smoke should never be significant enough to obscure the visibility of other road users, nor should it cause soot to gather on the rear of the vehicle.

What happens if my vehicle fails a roadside emission spot check?

You may be issued a notice to rectify defects, a prohibition preventing further driving, a £30 fixed penalty, or a court fine up to £1000 (£2,500 for Goods Vehicles/Buses). More severe penalties apply for tampering or if the vehicle poses a danger.

How do the May 2018 MOT rules affect diesel smoke tests?

The rules became much stricter. DPF-equipped cars must show no visible smoke and are tested against manufacturer plate values or stringent default limits. Non-DPF cars are also subject to stricter limits, based on plate values or new default figures, making it harder for older vehicles to pass than before.

Where can I find my vehicle's manufacturer emission plate value?

This figure is usually within a square box on the VIN plate in the engine bay or on a sticker located on the inner door pillar/sill or in the boot.

Will my car fail the MOT if the Engine Management Light is on?

Yes, as of May 2018, if your Engine Management Light (MIL) is illuminated at the time of the test, it will result in an automatic fail. The test also checks if the light illuminates and extinguishes correctly.

Staying informed about vehicle smoke emissions and MOT requirements is paramount for all diesel vehicle owners in the UK. Regular maintenance, prompt attention to warning lights, and adherence to these standards will not only help you avoid penalties but also contribute to cleaner air and safer roads for everyone. Understanding the nuances of the metered diesel smoke test and the significant 2018 MOT changes is key to ensuring your vehicle remains roadworthy and compliant.

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