Common MOT Failures and How to Fix Them - UK Guide

Around 40% of vehicles fail their MOT test, but the majority of failures are caused by minor, easily fixable issues. Many of these problems can be identified and resolved before your test, saving you the inconvenience and cost of a retest.

This guide covers the most common MOT failure reasons, explains how to fix each one yourself where possible, provides cost estimates for repairs, and gives you a comprehensive pre-MOT checklist to help you pass first time.

What this guide covers:

• Most common MOT failure reasons
• DIY fixes for each issue
• Cost estimates for repairs
• Comprehensive pre-MOT checklist
• What to do if you fail
• Understanding the new MOT categories

Understanding MOT Failure Categories

Since May 2018, MOT defects are categorised as:

Dangerous Vehicle is unsafe to drive. It is illegal to drive the vehicle until repaired. Automatic fail.

Major Significant defect that may affect safety, environment, or put other road users at risk. Automatic fail.

Minor Small defect that does not significantly affect safety. Pass with advisory - repair recommended.

Advisory Items to monitor - may become failures in future. Pass with advisory.

Pass No defects found or only minor/advisory items.

Most Common MOT Failures

According to DVSA data, these are the most frequent failure reasons:

1. Lighting and signalling (30% of failures)
2. Suspension (21%)
3. Brakes (18%)
4. Tyres (10%)
5. Driver's view of the road (9%)
6. Emissions (7%)
7. Steering (3%)
8. Other (2%)

Let us examine each category and how to address the issues.

Lighting and Signalling Failures

Common Issues

Bulb failures

• Headlight bulb blown
• Tail light not working
• Brake light out
• Indicator bulb failed
• Number plate light not working

Lens damage

• Cracked or broken lens
• Condensation inside light unit
• Faded or discoloured lens

Alignment issues

• Headlights incorrectly aimed
• After headlight replacement without realignment

Warning lights

• Engine management light on
• ABS warning light illuminated
• Airbag light on

DIY Fixes

Replacing bulbs

Most bulb replacements are simple DIY tasks.

Skill level: Beginner | Time: 10-30 minutes | Cost: 5-30 pounds

General process:

1. Identify failed bulb (have someone operate lights while you observe)
2. Access bulb holder (varies by vehicle - check manual)
3. Remove old bulb (twist, pull, or unclip)
4. Insert new bulb (do not touch glass of halogen bulbs)
5. Test operation

Headlight bulbs:

• Often accessed from engine bay
• Remove dust cap, disconnect wiring
• Release spring clip, remove bulb
• Insert new bulb without touching glass

Tail lights:

• Usually accessed from inside boot
• Remove trim panel
• Twist bulb holder anti-clockwise
• Pull out and replace bulb

Tip: If one bulb has failed, the opposite side is likely to fail soon. Consider replacing in pairs.

Clearing condensation:

• Minor condensation may clear naturally
• Significant water ingress requires seal repair
• Check drain holes are not blocked
• May need replacement if seal has failed

Cost comparison:

Repair	DIY Cost	Garage Cost
Headlight bulb	5-25	20-50
Tail light bulb	3-10	15-30
Number plate bulb	2-5	10-20
Headlight alignment	Free-30	30-50

When to Use a Professional

• HID or LED headlight systems
• Headlight alignment after replacement
• Adaptive lighting system faults
• Warning light diagnosis

Learn more about warning lights

Suspension Failures

Common Issues

Worn shock absorbers

• Excessive bouncing after bumps
• Nose dive when braking
• Uneven tyre wear
• MOT test: Push down on each corner - excessive bouncing fails

Worn bushes

• Knocking noises over bumps
• Imprecise steering
• Often visible cracks in rubber

Damaged springs

• Broken coil spring
• Sagging ride height
• Clunking over bumps

Worn ball joints and track rod ends

• Play in steering
• Knocking when turning
• Uneven tyre wear

Corroded components

• Excessive rust on structural parts
• Weakened mounting points

DIY Fixes

Skill assessment: Most suspension work is intermediate to advanced level. However, some inspections and minor work can be done at home.

Visual inspection (Beginner):

1. Park on level ground
2. Look at vehicle stance - uneven ride height indicates problem
3. Check for obvious damage or corrosion
4. Look for oil leaking from shock absorbers
5. Check rubber boots and bushes for cracks

Bounce test (Beginner):

1. Push down firmly on each corner of the car
2. Release and count bounces
3. Should stop after 1-2 bounces
4. More bouncing indicates worn shock absorbers

Drop link replacement (Intermediate): Drop links (anti-roll bar links) are a common failure point.

Time: 30-60 minutes per side | Cost: 15-40 pounds per link

1. Safely jack and support vehicle
2. Remove wheel
3. Locate drop link (connects anti-roll bar to suspension)
4. Remove securing nuts (may need two spanners)
5. Remove old link
6. Fit new link, ensuring correct orientation
7. Tighten to specification

Cost estimates:

Component	DIY Parts	Garage Complete
Drop links (pair)	25-60	80-150
Track rod end	20-40	80-120
Shock absorbers (pair)	60-150	200-400
Ball joint	20-50	100-180
Coil spring	30-80	120-200

When to Use a Professional

• Coil spring replacement (spring compressors are dangerous)
• Shock absorber replacement (often needs special tools)
• Ball joint replacement (affects steering geometry)
• Any work requiring wheel alignment afterwards

Brake Failures

Common Issues

Worn brake pads

• Less than 1.5mm friction material remaining
• Wear indicator showing
• Metal-to-metal contact

Worn brake discs

• Below minimum thickness (stamped on disc)
• Excessive scoring or grooves
• Significant lip on disc edge

Brake fluid issues

• Contaminated fluid
• Low fluid level
• Old fluid failing boiling point test

Handbrake ineffective

• Does not hold vehicle
• Excessive travel
• Imbalanced operation

Brake pipes and hoses

• Corrosion on rigid brake pipes
• Cracked or bulging flexible hoses
• Evidence of leaks

DIY Fixes

Brake pad inspection and replacement

See our detailed guide: How to Replace Brake Pads

Skill level: Intermediate | Time: 1-2 hours per axle | Cost saved: 60-120 pounds

Checking brake fluid:

Skill level: Beginner | Time: 5 minutes

1. Locate brake fluid reservoir (usually near bulkhead)
2. Check level is between min and max marks
3. Check fluid colour - should be light yellow, not dark brown
4. Dark fluid indicates it needs changing

Brake fluid change:

Skill level: Intermediate | Time: 30-45 minutes | Cost saved: 30-50 pounds

1. Remove old fluid from reservoir with syringe
2. Refill with fresh fluid (correct specification - usually DOT 4)
3. Bleed each caliper starting from furthest from reservoir
4. Pump brake pedal while assistant opens bleed nipple
5. Continue until clear fluid emerges
6. Repeat for each wheel
7. Check pedal firmness

Handbrake adjustment:

Many vehicles allow basic handbrake adjustment:

1. Locate adjuster (often inside cabin under trim, or at handbrake lever)
2. Turn adjuster to take up slack
3. Handbrake should engage within 3-5 clicks
4. Ensure wheels can still turn freely with handbrake off

Cost estimates:

Repair	DIY Cost	Garage Cost
Brake pads (front)	25-60	100-180
Brake pads (rear)	20-50	80-150
Brake discs (pair)	40-100	150-300
Brake fluid change	10-15	50-80
Handbrake adjustment	Free	30-50

When to Use a Professional

• Brake disc replacement (unless experienced)
• Brake pipe replacement
• ABS system faults
• Anything you are unsure about - brakes are safety critical

Tyre Failures

Common Issues

Insufficient tread depth

• Legal minimum: 1.6mm across central three-quarters
• Penalty: Up to 2,500 pounds and 3 points per tyre

Damage

• Cuts exposing cords
• Bulges in sidewall
• Cracks in rubber

Wrong specification

• Mixed tyre types on same axle
• Incorrect load or speed rating
• Unsuitable tyres for vehicle

DIY Fixes

Checking tread depth (Beginner):

20p test:

1. Insert 20p coin into tread grooves
2. If outer rim is visible, tread is below 1.6mm legal limit
3. Check across full width of tyre
4. Check in multiple places around tyre

Tread depth gauge: More accurate than 20p test

• Available from 2-5 pounds
• Insert into groove and read depth
• Replace tyres at 2-3mm for safety (do not wait until 1.6mm)

Checking for damage:

1. Visually inspect each tyre
2. Look for cuts, cracks, and bulges
3. Check sidewalls carefully
4. Feel for embedded objects
5. Look for uneven wear (indicates suspension/alignment issues)

Tyre replacement:

While physically changing a tyre is simple, fitting new tyres requires:

• Tyre removal from rim
• Balancing
• Proper fitting
• TPMS sensor handling (where fitted)

This is best done by a tyre fitting specialist.

Cost estimates:

Tyre Size	Budget	Mid-range	Premium
185/65R15	40-50	60-80	90-130
205/55R16	50-70	80-110	120-170
225/45R17	60-80	90-130	140-200
Fitting	10-15	10-15	10-15

Tip: Budget tyres often have shorter life and worse wet performance. Mid-range offers best value for most drivers.

When to Use a Professional

• Tyre fitting (requires specialist equipment)
• Wheel alignment after new tyres
• TPMS sensor issues
• Run-flat tyre replacement

Driver's View Failures

Common Issues

Windscreen damage

• Chips or cracks in driver's line of sight (Zone A - swept by wipers, in front of driver)
• Damage larger than 10mm in Zone A fails
• Damage larger than 40mm elsewhere in swept area fails

Wiper issues

• Worn blades leaving smears
• Blades not making contact
• Wiper arm damage

Mirror damage

• Cracked or missing mirrors
• Excessive movement
• Poor reflective surface

DIY Fixes

Windscreen chips:

Skill level: Beginner | Time: 15-30 minutes | Cost: 10-15 pounds

Small chips (smaller than 10p coin) can often be repaired:

1. Clean area thoroughly
2. Use windscreen repair kit (available from Halfords, Amazon)
3. Apply resin as per instructions
4. Cure with sunlight or UV lamp
5. Level with scraper

Cost comparison:

• DIY kit: 10-15 pounds
• Professional repair: 30-60 pounds
• Windscreen replacement: 100-400 pounds (often free via insurance)

Note: If chip is in Zone A (directly in driver's view), professional repair or replacement is recommended for MOT.

Wiper blade replacement:

Skill level: Beginner | Time: 5-10 minutes | Cost: 15-50 pounds

Covered in our DIY repair guide

Mirror replacement:

Wing mirrors are often DIY replaceable:

1. Remove door trim (if accessing from inside)
2. Disconnect electrical connectors (powered mirrors)
3. Remove mounting bolts/nuts
4. Fit replacement mirror
5. Reconnect and test

Cost: 20-100 pounds for aftermarket mirror, 50-200+ for genuine

When to Use a Professional

• Windscreen replacement
• Chips/cracks in Zone A
• Complex mirror mechanisms
• Heated/folding mirror faults

Emissions Failures

Common Issues

Excessive emissions

• Failed emissions test
• Diesel smoke test failure
• Petrol catalyst efficiency

Engine management light on

• Automatic MOT failure since 2018
• Indicates emissions-related fault codes stored

Visible smoke

• Blue smoke (burning oil)
• Black smoke (running rich)
• White smoke (coolant leak or condensation)

DIY Fixes

Before MOT emissions check:

Get the car warm:

• Drive for 20-30 minutes before test
• Include some motorway/dual carriageway driving
• Warm catalytic converter works more efficiently
• Engine at operating temperature runs cleaner

Fresh oil:

• Ensure oil is clean and at correct level
• Old, dirty oil can increase emissions
• Consider oil change if overdue

Air filter:

• Dirty air filter increases emissions
• Replace if dirty (see DIY guide)
• Cost: 8-25 pounds

Fuel additives:

• Catalytic converter cleaner can help borderline cases
• Pour into tank before filling with fuel
• Drive for 30+ miles before test
• Cost: 10-20 pounds

For diesels - DPF regeneration:

• Take car for extended drive before MOT
• 30+ minutes at motorway speeds
• Allows DPF regeneration to clear soot
• Critical for passing diesel smoke test

Addressing engine management light:

This requires diagnosis:

1. Use OBD-II reader to retrieve fault codes (20-50 pounds to buy, or free at some garages)
2. Research codes online for your vehicle
3. Common causes include:
   • Faulty oxygen sensor
   • Loose fuel cap
   • Failing catalytic converter
   • Mass airflow sensor
   • Ignition coil failure

Some codes clear themselves after repair. Others require clearing with diagnostic tool.

Cost estimates:

Repair	DIY Cost	Garage Cost
Air filter	10-25	30-50
Spark plugs	20-50	80-150
Oxygen sensor	30-80	100-200
Catalytic converter	150-400	400-1000+
DPF cleaning	100-200	200-400

When to Use a Professional

• Engine management light diagnosis
• Catalytic converter issues
• DPF problems that do not clear
• Complex emission system faults

Pre-MOT Checklist

Use this checklist 1-2 weeks before your MOT to identify and fix issues:

Lights (Check all function)

• Headlights (dipped and main beam)
• Front fog lights (if fitted)
• Rear fog lights
• Indicators (front, rear, side)
• Hazard warning lights
• Brake lights (all)
• Reversing lights
• Number plate lights
• Dashboard warning lights (should all be off)

Tyres

• Tread depth minimum 1.6mm (check multiple points)
• No cuts, bulges, or damage
• Correct type on each axle
• Correct inflation

Brakes

• Brake pedal firm, not spongy
• Does not pull to one side
• Handbrake holds vehicle
• Brake fluid level correct
• No warning lights

Suspension and Steering

• No excessive bouncing
• No knocking noises
• Steering not excessively loose
• No power steering warning

Visibility

• Windscreen free of significant damage
• Wipers clear effectively
• Washers work
• All mirrors present and secure

Other

• Horn works
• Seat belts function
• Doors open and close properly
• Fuel cap seals
• Registration plates secure and readable

What to Do If You Fail

Understand the Failure

1. Read the failure sheet carefully - It lists all defects and their category
2. Ask the tester to explain - They can show you the issues
3. Photograph problems - Useful for getting repair quotes

Your Options

Repair and retest at same station:

• Partial retest free if within 10 working days
• Same items retested only
• Must be same testing station

Repair elsewhere:

• Can take to your preferred garage
• Full retest fee applies (around 30-50 pounds)
• Useful if you have trusted mechanic

Leave the car:

• If dangerous defect, you cannot legally drive it
• Major defect: Driving it is illegal (technically)
• Must be trailered or repaired on site

Challenging the Result

If you believe the failure is incorrect:

1. First discuss with the testing station
2. If unresolved, contact DVSA
3. DVSA can arrange re-examination
4. Must be requested within 14 days

Cost Management

Get multiple quotes:

• Failure sheet is standardised - any garage can quote from it
• Prices vary significantly
• Balance cost against convenience

Prioritise repairs:

• Safety items must be fixed
• Some advisories can wait
• Focus on items needed to pass

Summary

Most MOT failures are preventable with basic checks and maintenance:

1. Check lights regularly - Bulb failures are the top cause
2. Monitor tyre condition - Easy to check, serious if failed
3. Listen to your car - Unusual noises indicate problems
4. Address warning lights - Engine management light is automatic failure
5. Get your car warm - Emissions are better with warm engine
6. Use the pre-MOT checklist - Catch issues before the test

A little preparation saves time, money, and the inconvenience of a failed test. Many fixes are simple DIY tasks; for others, getting professional help early is more cost-effective than failing and retesting.

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