Vehicle maintenance

Drivers and fleet operators have a responsibility to ensure their vehicles are roadworthy and well-maintained. If safety critical parts such as brakes and wheels are not kept in good repair, this could cause the driver to lose control or fail to respond in time in an emergency, with potentially fatal results. Vehicle defects contributed to 2,000 crashes in 2013, 42 of them causing deaths [1].

In the UK, all cars, motorbikes and light passengers vehicles must pass an annual MOT test once they are three years old. Trucks and buses have their own version of the MOT, known as the annual vehicle test. These tests are intended to confirm that vehicles meet roadworthiness and environmental standards. If vehicles are kept well-maintained throughout the year, with any defects noticed and remedied promptly, they should have no trouble passing the test. However, many drivers do not pay enough attention to routine maintenance: 40% of cars and vans failed their initial MOT in 2012/13 [2].

As well as putting drivers and others around them at risk, if defects are not dealt with promptly it can cost drivers money, such as by increasing fuel use and/or the overall cost of repairs, and can increase pollution.

Below are some of the most common safety-critical defects in vehicles, and how they can be avoided.

Learn more: Read our advice for drivers on maintaining your vehicle.

Incorrect tyre pressure and tread depth

Illegal, defective or under-inflated tyres is the most common vehicle defect contributing to fatal crashes [3], yet is one of the simplest to detect and rectify. Many drivers do not pay attention to the state of their tyres: a Brake survey found seven in 10 UK drivers (70%) did not know the legal minimum tyre tread depth for their vehicle [4]. A separate survey of young drivers found one in three (34%) never check their tyre tread depth and one in four (25%) never check tyre pressure [5].

Tyres must be kept in at the correct pressure as recommended by the vehicle manufacturer. Driving with under-inflated tyres results in reduced steering control, longer stopping distances, increased risk of skidding on wet roads, greater fuel consumption, and more wear to the tyres [6]. Over-inflated tyres are more susceptible to damage, and will wear more quickly in the middle [7]. Drivers should check their tyre pressures at least once a month using a pressure gauge, and before every long journey.

Tyres must be replaced when the tread depth wears down, as this greatly increases stopping distances. In the UK car and van tyres must be replaced before the legal minimum tread depth of 1.6mm, however research has found that braking distances increase far before this point, particularly in wet conditions [8]. Brake therefore recommends that tyres are changed once tread depth reaches 3mm. Drivers should check tyre tread depth once a week, and before every long journey.

Tyres must also be kept in good condition, with drivers looking out for any visible signs of wear, cracks and bulges and replacing tyres if they spot any problems.

Tyres can still age, even when not in use. Tyre rubber deteriorates over time, becoming brittle and developing cracks. Drivers should therefore have tyres inspected by a tyre specialist at least once a year once they are more than five years old, as deterioration may not be immediately visible. All tyres, including spares, should be replaced before they reach ten years old, even if they are not worn [9].

Loose wheels

If a wheel becomes detached while the vehicle is moving it can accelerate up to 150km/h (93mph) [10], and would hit an oncoming vehicle, pedestrian or cyclist with incredible force. It can also cause the driver who has lost the wheel to lose control and crash. It’s estimated that several hundred wheel detachments occur each year in the UK, mostly involving large commercial vehicles such as trucks and buses. One in six (16%) wheel loss incidents causes injury, and one in 24 (4%) causes death [11].

There are several reasons a wheel can become detached from a vehicle:

  • the studs and bolts attaching the wheel can become loose, due to under-tightening, or break, due to over-tightening
  • incorrect components may have been used (not all wheel fixings and wheels are compatible)
  • components may be in poor condition or of poor quality
  • rust or dirt may build up on wheel fixings, causing damage over time.

Drivers should therefore look out for:

  • rust or bright metal around the wheel nut
  • cracked or distorted wheel rims
  • broken or loose fixings.

Wheel nut indicator devices, which can indicate if wheel nuts have moved, are available from various manufacturers. They are recommended for heavy vehicles such as trucks or buses, and can also be used on cars and vans. If your vehicle has these fitted, you should check their position when carrying out other essential checks such as tyre pressure and tread depth.

Learn more: Organisations with vehicle fleets can access guidance on vehicle maintenance at

Worn brakes

Brakes are one of the most important safety components of a vehicle: without properly working brakes, you will have no way of stopping in time to avoid a crash. Defective brakes are the most common vehicle defect contributing to serious crashes, and the second most common (after defective tyres) contributing to fatal crashes [12].

Regularly checking brakes is important to drivers noticing any problems early on and getting them fixed, avoiding more serious and costly problems and the deadly risk of sudden brake failure.

Drivers should look out for any of the following warning signs [13]:

  • needing to press much harder than usual on the brake
  • the vehicle taking longer than usual to stop when brakes are applied
  • the brake pedal sinking right down when you put your foot on it
  • the brake pedal becoming very stiff and hard to push down
  • the handbrake not releasing, or moving up and down much easier than usual.

It is important for drivers to test brakes regularly to help ensure they notice any differences. This should be done on a safe, empty, flat stretch of road, early in the journey, once a week and before long journeys. If drivers notice any problems, or are in any doubt as to the state of brakes, they should consult a qualified mechanic immediately and not risk driving a vehicle if there is a possibility the brakes are damaged.

Other defects

Defective lights, indicators, steering, suspension, missing or defective mirrors, and overloaded vehicles, also contribute to crashes [14]. Drivers should check these components and those above on a regular basis using ‘walk-round’ checks. See Brake’s advice for drivers.

[1] Reported road casualties Great Britain: annual report 2013, Department for Transport, 2014

[2] VOSA effectiveness report 2012 to 2013, Driver and Vehicle Standards Agency, 2014

[3] Reported road casualties Great Britain: annual report 2013, Department for Transport, 2014

[4] Are you ready to drive? Brake and Direct Line, 2009

[5] Young drivers and their tyres, TyreSafe and Ingenie, 2013

[6] Care guide: tyre pressures, Michelin, undated

[7] Air Pressure - Correct, Underinflated and Overinflated,, undated

[8] Tyres should be changed at 3mm, RoadSafe, Mira, Continental, 2005

[9] When should I change my tyres? Michelin, undated

[10] Wheel security: a best practice guide, Society of Operations Engineers, 2009

[11] Heavy vehicle wheel detachment, Transport Research Laboratory, 2006

[12] Reported road casualties Great Britain: annual report 2013, Department for Transport, 2014

[13] Faulty brakes,, 2013

[14] Reported road casualties Great Britain: annual report 2013, Department for Transport, 2014

Page last updated: October 2014

Tags: fleet vehicle maintenance