Diesel Cars & Their First MOT: What to Watch For

When your car reaches its third birthday, a significant milestone looms on the horizon: its first annual MOT test. This crucial assessment ensures your vehicle is roadworthy and safe for continued use on UK roads. While many owners might expect their relatively new cars to sail through this initial inspection, recent data suggests a surprising number of models, including many popular diesel variants, are far from guaranteed a pass. Indeed, analysis of cars registered in 2016 and undergoing their first MOT in 2019 reveals that some vehicles have as much as a one in four chance of failing straight away.

This comprehensive review delves into the findings from motoring experts, highlighting the specific diesel models that struggled the most and, more importantly, the prevalent reasons behind these failures. Understanding these common pitfalls is key to ensuring your diesel car is well-prepared for its vital roadworthiness check, potentially saving you time, money, and the inconvenience of a re-test.

The Evolving MOT Landscape for Diesel Vehicles

The MOT test is a cornerstone of road safety in the UK, adapting over time to incorporate new technologies and environmental concerns. A significant update occurred in May 2018 when the Driver and Vehicle Standards Agency (DVSA) introduced new assessment criteria, rules, and defect categories. These changes were particularly impactful for diesel vehicles, bringing in tougher emission requirements and increased scrutiny of components like the diesel particulate filter (DPF). Defects are now categorised as Minor, Major, or Dangerous, with Major and Dangerous faults resulting in an automatic fail.

While the focus on emissions for diesels is undoubtedly stricter, the data from KnowYourCar, a motoring website and app that crunched the numbers for 2016-registered vehicles, reveals that many first-time failures, even for diesel cars, are often down to surprisingly simple, yet critical, issues. As you will see, problems with basic visibility and lighting frequently top the list of reasons for a failed MOT.

Common Culprits: Headlights and Wipers

It might seem baffling that issues as seemingly minor as headlight alignment or worn windscreen wipers could cause a new car to fail its MOT, but the data is clear. Dan Owen from KnowYourCar expressed his astonishment at the volume of fails attributed to these problems. Misaligned headlights, for instance, can be caused by anything from minor bumps to factory assembly issues, and critically, they can dazzle oncoming drivers, posing a significant safety risk. Similarly, windscreen wipers that fail to adequately clear the screen impair the driver's view, which is a direct safety concern.

While the data doesn't always group individual fail reasons, broader categories like 'Lights and Indicators' (29.5%), 'Suspension' (20.2%), and 'Brakes' (17.2%) are the most common reasons for MOT failures across all vehicles, regardless of age. However, for these younger cars undergoing their first test, specific issues like headlamp alignment and windscreen wipers are highly prevalent. For diesel cars, the added scrutiny on emissions and DPFs also plays a role, though often not as dominant as these more straightforward mechanical or maintenance issues.

Diesel Cars Most Likely to Flunk Their First MOT

Let's dive into the specific diesel models that registered the lowest pass rates for their first MOT tests in 2019. This list is based on models where 500 or more cars were tested in the last year, providing a robust dataset for analysis. It’s important to remember that a low pass rate doesn't necessarily mean a car is inherently unreliable; often, it points to common, rectifiable issues that owners might overlook.

20. Renault Grand Scenic (Diesel)

First MOT Pass Rate: 84.21%
Number of cars taking first MOT test in 2019: 1,520

Despite the declining popularity of MPVs in favour of SUVs, over 1,500 diesel Grand Scenics from 2016 underwent their first MOT. A significant 15% of these failed, with the primary culprit being poor headlight alignment. This suggests that while the car might be mechanically sound, a simple check and adjustment of its lights could have prevented many failures.

18. Toyota Hilux (Diesel)

First MOT Pass Rate: 83.92%
Number of cars taking first MOT test in 2019: 5,602

The Toyota Hilux has an almost legendary reputation for durability, famously surviving extreme tests. However, its first MOT pass rate of under 84% suggests that even robust vehicles aren't immune to common MOT pitfalls. Headlamp alignment was again the most common cause of failure, affecting 5% of tested Hiluxes. This highlights that even for a workhorse vehicle, basic checks are paramount.

17. Volkswagen Passat (Diesel)

First MOT Pass Rate: 83.90%
Number of cars taking first MOT test in 2019: 18,978

A popular choice for company car drivers, almost 19,000 diesel Passats from 2016 faced their first MOT. Nearly 16% of these failed, with the most common reason being 'windscreen wiper does not clear the windscreen effectively'. This issue alone accounted for 11% of all failures, underscoring how crucial clear visibility is for a pass.

16. SsangYong Korando (Diesel)

First MOT Pass Rate: 83.81%
Number of cars taking first MOT test in 2019: 1,254

The budget-friendly SsangYong Korando SUV also struggled, with almost 16% of tested diesel models failing their first MOT. Similar to other vehicles on this list, the main reasons were prosaic: windscreen wiper problems and headlight alignment issues. This indicates a consistent theme across various makes and models.

14. Vauxhall Corsa (Diesel)

First MOT Pass Rate: 83.07%
Number of cars taking first MOT test in 2019: 5,405

While petrol Corsas are far more common, nearly 5,500 diesel variants from 2016 underwent their first MOT. Only 83% passed. Once more, windscreen wipers failing to clear the screen correctly was the dominant issue, impacting 6% of all diesel Corsas scrutinised. This is a simple fix that could save many owners a re-test fee.

13. Volkswagen Touran (Diesel)

First MOT Pass Rate: 82.58%
Number of cars taking first MOT test in 2019: 3,629

Another large MPV to feature, the diesel VW Touran saw 13% of its 2016-registered models fail their first MOT due to window wiper problems. The recurring nature of this fault across different MPVs and vehicle types suggests that wiper blade condition is a frequently overlooked aspect of preventative maintenance.

12. Citroen C4 Cactus (Diesel)

First MOT Pass Rate: 82.49%
Number of cars taking first MOT test in 2019: 1,382

Both petrol and diesel versions of the quirky C4 Cactus appear on this list. For the diesel variant, approximately 17% failed. The same common issues plagued these oil burners: windscreen wipers and headlight alignment. This is good news in a way, as it suggests that the stricter diesel emission and DPF checks introduced in 2018 weren't the primary cause of failure for these particular models.

11. Citroen C4 Grand Picasso (Diesel)

First MOT Pass Rate: 82.37%
Number of cars taking first MOT test in 2019: 3,188

As a seven-seat MPV, the C4 Grand Picasso is built for family transport. However, just over 82% of its diesel versions passed their first MOT. While headlamp aim being out of alignment was prominent, an interesting second common issue was with the 'registration lamp' that illuminates the rear number plate, impacting 3% of all models tested. This highlights the importance of checking all lights, not just the main ones.

9. Ford Fiesta (Diesel)

First MOT Pass Rate: 81.74%
Number of cars taking first MOT test in 2019: 13,163

The UK's long-reigning best-seller, the Ford Fiesta, also features on this list, specifically its diesel variant. Of the 13,000 diesel Fiestas from 2016, fewer than 82% passed their first MOT without issues. Headlamps out of alignment were responsible for 5% of failures, but notably, 327 cars (2%) failed due to exhaust emission problems. This is one of the clearer indicators that diesel-specific checks are indeed catching out some vehicles.

6. Dacia Logan (Diesel)

First MOT Pass Rate: 81.84%
Number of cars taking first MOT test in 2019: 2,225

The Dacia Logan diesel struggled even more than its petrol counterpart, with almost one in five failing their first MOT. The common issues of unfit windscreen wipers and misaligned headlights were consistently recorded for these diesel models, reinforcing the trend of basic maintenance oversights.

5. Isuzu D-Max (Diesel)

First MOT Pass Rate: 81.46%
Number of cars taking first MOT test in 2019: 5,221

This robust pick-up, popular with small businesses, saw around 18% of its 2016 diesel models fail their first MOT. Despite its rugged appearance, it was susceptible to the same issues plaguing other cars: headlamp aim out of alignment was a significant factor, affecting 4% of all tested vehicles.

4. Fiat 500L (Diesel)

First MOT Pass Rate: 80.88%
Number of cars taking first MOT test in 2019: 1,334

Fiat's attempt at a practical MPV using the iconic 500 name didn't fare well, with an almost 20% first MOT failure rate for its diesel variants. Again, wiper issues were the most common reason for these failures, highlighting a persistent problem across various manufacturers.

3. Dacia Sandero (Diesel)

First MOT Pass Rate: 79.03%
Number of cars taking first MOT test in 2019: 4,854

The Dacia Sandero diesel had an even worse record than its petrol sibling, with more than one in five failing their first MOT. Headlights out of alignment accounted for 12% of all diesel Sanderos tested failing, while unfit wipers and failing windscreen washers also contributed significantly. This points to a pattern of basic pre-MOT checks being missed.

2. Peugeot 5008 (Diesel)

First MOT Pass Rate: 77.15%
Number of cars taking first MOT test in 2019: 2,696

Another MPV with a poor track record, the diesel Peugeot 5008 had an almost 23% failure rate for its first MOT. The most common issues were, predictably, headlight alignments, but also a notable failure of brake lights, impacting 11% of all 5008s tested. This demonstrates the importance of checking all signalling lights.

1. SsangYong Rodius (Diesel)

First MOT Pass Rate: 76.87%
Number of cars taking first MOT test in 2019: 709

Topping the list as the diesel car most likely to fail its first MOT was the SsangYong Rodius MPV. A worrying 23% of the 709 examples tested were deemed unfit for the road. The most common reasons were windscreen wipers not being in a fit state to clear the screen (6%) and headlights out of alignment (5%). This highlights that even for the worst performers, the issues are often straightforward maintenance points.

Summary of Top Diesel First MOT Failures (2016 Registrations)

What Can Diesel Car Owners Do?

The data clearly shows that many first MOT failures, even for diesel cars, are not due to complex engine problems or DPF issues alone, but rather simple, overlooked maintenance points. Preparing your car for its MOT doesn't have to be daunting. A proactive approach can significantly increase your chances of a first-time pass.

• Check All Lights: This includes headlights (high and low beam), brake lights, indicators, fog lights, reverse lights, and number plate lights. Get someone to help you check brake lights.
• Inspect Windscreen Wipers: Ensure they clear the screen effectively without streaking. Replace them if they're worn or damaged. Also, check your washer fluid level.
• Tyres: Check tyre pressure, tread depth (minimum 1.6mm across the central three-quarters of the tread and around the entire circumference), and for any damage, bulges, or cuts.
• Fluid Levels: Ensure brake fluid, power steering fluid, and screen wash are topped up to the correct levels.
• Brakes: While a full inspection requires specialist tools, check your handbrake operation and listen for any unusual noises when braking.
• Horn: Give it a quick blast to ensure it works.
• Mirrors: Check that all mirrors are secure and undamaged.
• Seatbelts: Pull each seatbelt to its full extension, checking for fraying or damage, and ensure the buckle clicks securely.
• Dashboard Warning Lights: If any warning lights are illuminated on your dashboard (e.g., engine management, ABS, airbag), your car will fail. Address these before the test.
• DPF and Emissions for Diesel: While the common issues are often simpler, be mindful of diesel-specific checks. If you drive short distances frequently, consider a longer, higher-rev drive before your MOT to help clear the DPF. If you notice excessive smoke of any colour from the exhaust, or suspect DPF tampering, get it checked by a professional.

Frequently Asked Questions (FAQs)

Why are so many relatively new diesel cars failing their first MOT?

While stricter diesel emission rules were introduced in 2018, the data suggests that for many 2016-registered diesel cars, the primary causes of first MOT failures are often surprisingly basic maintenance issues, such as misaligned headlights, worn windscreen wipers, and faulty brake lights, rather than complex engine or DPF problems. These are often overlooked by owners who assume a newer car will naturally pass.

Are diesel cars more likely to fail the MOT than petrol cars?

The provided data highlights specific diesel models with low first MOT pass rates. While there are stricter emission checks for diesels, the analysis shows that common mechanical and visibility issues (like lights and wipers) are still the dominant reasons for failure across both fuel types, rather than a blanket higher failure rate for diesels solely due to their engine type. The list features both petrol and diesel cars in its full compilation, indicating issues aren't exclusive to one fuel type.

What is the most common reason for a diesel car to fail its first MOT?

Based on the data for 2016-registered diesel cars undergoing their first MOT in 2019, the most common reasons for failure consistently revolve around headlamp alignment and defective windscreen wipers. These issues directly impact visibility and safety, making them critical failure points despite their apparent simplicity.

Conclusion

The first MOT test can be a nerve-wracking experience, but for owners of 2016-registered diesel cars, the data reveals a clear pattern: many failures are entirely avoidable. While tougher emissions checks are now in place for diesels, it's often the fundamental elements of car maintenance – properly aligned headlights, effective windscreen wipers, and functioning brake lights – that lead to a red cross on the MOT certificate. By conducting simple, proactive checks before your car's third birthday, you can significantly improve its chances of passing first time, ensuring your diesel remains safe, legal, and ready for the road ahead.

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