Tyre Pressure and MOT Failures

The MOT test is a crucial annual inspection designed to ensure that vehicles are safe and environmentally sound. Many drivers meticulously prepare their cars for this test, focusing on common failure points like brakes, lights, and emissions. However, one area that might be overlooked, yet can lead to a failed MOT, is tyre pressure. This article will delve into the specifics of how tyre pressure relates to MOT regulations in the UK, explaining why maintaining the correct inflation is vital for passing your test and, more importantly, for road safety.

Tyre Condition and the MOT Test

The MOT test has specific criteria for judging the condition of tyres. While a flat tyre is an obvious failure, the rules also cover aspects that might not be immediately apparent to the untrained eye. The Vehicle and Operator Services Agency (VOSA), now part of the Driver and Vehicle Standards Agency (DVSA), sets the standards for MOT testers. These standards are comprehensive and cover various aspects of tyre health, including tread depth, sidewall damage, and, crucially, the overall condition and suitability of the tyre for the vehicle.

Under-Inflated Tyres: A Direct Cause for Failure?

The direct answer to whether under-inflated tyres will fail an MOT is nuanced. While there isn't a specific line item in the MOT manual that states "under-inflated tyres fail," the consequences of under-inflation can certainly lead to a fail. Here's how:

1. Tyre Damage and Wear

When a tyre is under-inflated, the sidewalls flex excessively. This constant flexing generates heat, which can damage the tyre's internal structure. This damage can manifest as:

• Premature and uneven wear: The outer edges of the tread will wear down much faster than the centre. This is a direct failure point in the MOT test, as tyres must have a minimum tread depth of 1.6mm across the central three-quarters of the tread width. Excessive wear on the edges due to under-inflation will mean that part of the tread falls below this legal limit.
• Sidewall bulging or cracking: The constant stress on the sidewalls can cause them to bulge outwards or develop cracks. Any damage to the sidewall, including cuts, bulges, or distortion, is a major fail item in the MOT.
• Overheating and blowout risk: Excessive heat build-up can lead to the tyre material degrading, increasing the risk of a sudden blowout. While a blowout itself isn't tested, the underlying structural damage that makes it more likely can be identified.

2. Handling and Stability

Under-inflated tyres significantly affect a vehicle's handling and stability. The MOT test assesses the overall safety of the vehicle, and this includes how it performs on the road. Under-inflated tyres can lead to:

• Poor steering response: The vehicle may feel sluggish or unresponsive when steering.
• Increased braking distances: The contact patch of the tyre with the road is altered, potentially increasing the distance it takes to stop.
• Instability during cornering: The tyre may not support the vehicle's weight adequately during turns, leading to a feeling of instability or even the tyre rolling off the rim.

While a tester might not directly measure tyre pressure as part of the standard MOT check, they are trained to identify signs of tyre damage and wear that are *caused* by incorrect inflation. A tyre that is visibly deflated or shows signs of damage consistent with under-inflation will likely result in a failed MOT.

Legal Requirements and Tyre Pressures

The legal requirement for tyre tread depth is 1.6mm across the breadth of the tread in a continuous band. However, it's also a legal requirement for tyres to be "fit for purpose." This means they must be free from cuts, abrasions, bulges, and other defects that could compromise safety. Under-inflation, by causing these defects, directly contravenes the "fit for purpose" requirement.

Where to Find Your Vehicle's Correct Tyre Pressures

Every vehicle has a recommended tyre pressure, which is determined by the manufacturer based on the vehicle's weight, suspension, and intended use. This information is usually found in several places:

• Driver's side door pillar: A sticker here often details the correct pressures for the front and rear tyres, and sometimes for a fully loaded vehicle.
• Fuel filler cap: Some manufacturers place the tyre pressure information inside the fuel filler flap.
• Owner's manual: The vehicle's handbook will always contain this information, often in a dedicated section on tyres or maintenance.

It's important to note that tyre pressures should be checked when the tyres are cold, meaning the vehicle hasn't been driven for at least three hours or has only been driven for a very short distance at low speed. Driving heats up the tyres, increasing the air pressure inside and giving an inaccurate reading.

The Importance of Regular Tyre Checks

Given the potential consequences, regular tyre pressure checks are essential, not just for passing your MOT but for maintaining the safety and efficiency of your vehicle.

Benefits of Correctly Inflated Tyres:

• Improved fuel economy: Under-inflated tyres increase rolling resistance, meaning your engine has to work harder, consuming more fuel.
• Extended tyre life: Correct inflation ensures even wear across the tread, maximising the lifespan of your tyres.
• Enhanced handling and braking: Proper inflation is critical for optimal grip and control, leading to safer driving.
• Reduced risk of punctures and blowouts: Maintaining the correct pressure minimises stress on the tyre structure.

MOT Test: What Testers Look For Regarding Tyres

During an MOT test, a qualified tester will examine your tyres for several specific issues. These include:

It's important to understand that while a tester might not inflate your tyres, they are obligated to fail a vehicle if the tyres are in a condition that suggests they are unsafe due to under-inflation, such as excessive edge wear or sidewall damage. A tyre that is simply a little soft but otherwise in good condition might be a "minor" defect, but one that is significantly low and showing signs of stress will be a major concern.

Frequently Asked Questions

Q1: Can I fail my MOT just for having low tyre pressure?

A: Not directly. You will fail if the low tyre pressure has caused damage or excessive wear that contravenes MOT regulations, such as tread depth below 1.6mm on the edges, or visible sidewall damage like bulges or cracks.

Q2: How often should I check my tyre pressure?

A: It's recommended to check your tyre pressure at least once a month and before any long journeys. Many garages and petrol stations have tyre pressure gauges available.

Q3: What is the difference between a minor and a major tyre defect in an MOT?

A: A minor defect is something that doesn't immediately affect safety but should be addressed. A major defect is something that makes the vehicle unsafe or does not comply with the law, and will result in a fail.

Q4: Does the MOT test check if my tyres are the correct pressure?

A: No, the MOT test does not involve checking or adjusting tyre pressures. However, the *condition* of the tyres, which can be a result of incorrect pressure, is thoroughly checked.

Q5: My tyres look a bit soft, but they passed their MOT. Why?

A: The tester would only fail the vehicle if the low pressure had led to a defect that contravened the MOT standards. If the tyres were only slightly low and had no visible damage or excessive wear, they might pass.

Conclusion

While under-inflated tyres are not a direct MOT failure point in themselves, the detrimental effects they have on tyre condition and vehicle safety can very easily lead to a failed test. By ensuring your tyres are always inflated to the manufacturer's recommended pressures, you not only safeguard your vehicle against an MOT failure but also contribute to your own safety and the safety of other road users. Regular checks and maintaining the correct tyre pressure are simple yet vital aspects of vehicle ownership that should never be neglected.