Many drivers assume that giving their car a quick wash before an MOT test is always helpful. While cleanliness does matter, leftover residue from car wash products and everyday dirt buildup can actually interfere with MOT visibility checks. These small, often overlooked issues can lead to advisory notes or even failures, despite the vehicle being mechanically sound.

Understanding how visibility checks work and how residue affects them can help you prepare your car properly and avoid unnecessary MOT problems.

What Are MOT Visibility Checks?

MOT visibility checks focus on whether the driver has a clear, unobstructed view of the road. Testers assess components such as:

• Windscreen condition
• Wipers and washers
• Mirrors
• Lights and indicators
• Number plates

Anything that reduces visibility or makes key features hard to read or see can result in a fail or advisory.

How Car Wash Residue Causes Problems

Some car shampoos, waxes, and quick-detail sprays leave behind a thin film on glass and lights. While this may look shiny in daylight, it can cause glare or smearing under workshop lighting. Common issues include:

• Streaky windscreens that blur vision when washers are used
• Greasy residue on headlights, reducing light output
• Film on mirrors, affecting rear visibility

These issues are especially noticeable when testers activate washers or inspect lights closely.

The Impact of Dirt and Grime Buildup

On the other hand, dirt is just as problematic. Road grime, salt, and dust can accumulate in areas drivers often forget to clean, such as:

• Headlight lenses
• Rear lights and reflectors
• Number plates
• Camera or sensor areas (on newer vehicles)

Dull or obscured lights can reduce brightness, while dirty number plates may be considered unreadable—both potential MOT failures.

Windscreen Washers and Wipers: A Common Failure Point

If washer jets spray dirty water or leave smeared residue, visibility is compromised. Old washer fluid mixed with grime can worsen the problem. Similarly, wiper blades dragging residue across the glass may fail to clear the screen effectively, which is a common MOT fail item.

Simple Cleaning Tips Before Your MOT

To avoid visibility-related issues:

• Rinse thoroughly after washing to remove all shampoo residue
• Clean glass with a dedicated, residue-free glass cleaner
• Wipe headlights and mirrors with a microfibre cloth
• Top up washer fluid with proper screen wash
• Check wipers for smearing or wear

Before your test, you can also review your vehicle’s previous results using tools like the free MOT history checker at https://checkmot.com/ to spot recurring advisories.

Why Small Details Make a Big Difference

MOT testers must follow strict DVSA guidelines. Even minor visibility issues can’t be ignored. Spending a few extra minutes cleaning properly—not just quickly—can prevent avoidable fails and save you time and money.

FAQs

Can a dirty windscreen cause an MOT failure?

Yes. If dirt or residue significantly restricts the driver’s view, it can lead to a fail.

Do headlights need to be clean for an MOT?

Yes. Dirty or cloudy headlights can reduce light output and result in a fail or advisory.

Is wax on the windscreen a problem?

It can be. Wax often causes glare and smearing, which may affect visibility checks.

Should I wash my car right before an MOT?

Yes, but rinse thoroughly and clean glass properly to avoid leaving residue behind.

Can unreadable number plates fail an MOT?

Yes. Number plates must be clearly visible and legible to pass.