Article from: DVSA Matters of Testing,
1. Headlamp aim
The top fail item for all vehicle types is ‘headlamp aim incorrectly set’. If your headlamp or headlamp bulb has been replaced since the last test then it may be worth checking whether one headlamp is significantly higher than the other, or the aim isn’t too high or low. You can do this by shining the headlamps on a garage door or another suitable surface.
While checking the headlamps, ask someone to sit in the car to operate the rest of the lamps while you walk around the car checking all the other lamps. Don’t forget the hazard warning lamps and the number plate lights. If these don’t work, changing the bulb is usually simple enough, but some headlamp bulbs can be much more difficult and require assistance from a repairer.
2. Tyre tread depth
Another common fail for all vehicle types is tyre tread depth below the minimum requirement. If any of the tread looks low, insert a 20p coin edge into the main grooves and if the outer rim of the coin is hidden then the tread is probably fine. If you’re not sure, many tyre outlets are happy to do a free check.
3. Windscreen wipers
Windscreen wipers not clearing the windscreen effectively are another common fail. You can easily check this by operating the wipers and the washers. If the blades don’t clear the water effectively from the screen and leaves areas untouched or smears across the driver’s view then this may result in a fail. Wiper blades do wear over time and become damaged by grit and dirt, so they need replacing at regular intervals. Also if there isn’t any fluid in the washers then this would also be a fail.
The most common failures can be avoided by taking the time to do a simple walk around check of the vehicle before taking it for test. Potential failure items can be easily identified and rectified by owners.