Why Your Vehicle Might Fail an MOT Emissions Test – Common Causes Explained

At MPG Tuning Ltd, we often meet customers worried about their car failing its MOT due to emissions—and with good reason. Emissions failures are one of the most common MOT issues, often triggered by problems across the fuel system, engine, or exhaust.

Understanding the root cause is the first step to solving it. Here’s a detailed breakdown of the most frequent culprits behind MOT emissions test failures:

Fuel System Issues

Fuel Leaks
Leaks in the fuel system can cause the engine to run rich—burning more fuel than necessary—which increases carbon monoxide and hydrocarbon emissions.

Faulty Injectors or Carburettors
Poor fuel delivery results in incomplete combustion, which leads to excess emissions and poor engine performance.

Engine-Related Problems

Engine Misfires
A misfiring engine won’t burn fuel efficiently, releasing unburnt hydrocarbons and raising emissions levels beyond legal limits.

Excessive Oil Consumption
Burning oil is a sign of internal engine wear or damage, which can contribute significantly to high particulate and hydrocarbon emissions.

Dirty or Blocked Air Filter
Restricted airflow means a richer fuel mixture and inefficient combustion, both of which increase emissions.

Exhaust System Faults

Faulty Catalytic Converter
The catalytic converter is vital for reducing harmful emissions. When it’s clogged or not functioning, your car can easily fail the MOT.

Exhaust Leaks
Any leaks in the system can allow unfiltered gases to escape, preventing the catalytic converter or DPF from doing their job.

DPF (Diesel Particulate Filter) Blockages
In diesel engines, a blocked or failed DPF can dramatically raise particulate emissions, leading to an immediate fail.

Other Common Causes

Oxygen (Lambda) Sensor Faults
These sensors regulate the air/fuel mixture. If they send incorrect data to the ECU, your engine may run inefficiently, increasing emissions.

Faulty Mass Air Flow (MAF) Sensor
This sensor determines how much air is entering the engine. If readings are off, it can disrupt the fuel mix and lead to higher emissions.

Loose or Damaged Fuel Cap
Often overlooked, a damaged or loose fuel cap can let fuel vapours escape, especially in petrol vehicles—contributing to elevated emissions.

Need Help with Emissions Issues?

At MPG Tuning, we offer accurate diagnostics, custom ECU remapping, and proven solutions for DPF, EGR, and AdBlue systems. Whether your vehicle has already failed its MOT or you want to prevent problems ahead of time, we’re here to help.

Carbon Cleaning: Reduce Emissions and Prevent Future Problems
At MPG Tuning, we’re all about giving your engine the best chance to perform cleanly, efficiently, and reliably. One of the most effective services we offer to reduce emissions—and prevent costly repairs—is professional carbon cleaning.

Whether your vehicle is struggling with high emissions, feeling a bit sluggish, or you simply want to keep it in top condition, carbon cleaning is a smart and affordable step.

What Is Carbon Cleaning?
Over time, carbon deposits build up inside your engine—especially around the intake valves, turbo vanes, EGR valve, piston crowns, injectors, and DPF. These carbon buildups disrupt airflow and fuel delivery, leading to poor combustion, higher emissions, and reduced engine efficiency.

Carbon cleaning is a non-invasive process that safely breaks down and removes these deposits, restoring your engine's performance and improving emissions output.

Benefits of Carbon Cleaning

Lower Emissions
Carbon build-up leads to incomplete combustion, increasing harmful gases like CO, HC, and NOx. By cleaning internal engine components, you improve combustion efficiency and reduce these pollutants—helping your vehicle pass the MOT emissions test.

Better Fuel Economy
A clean engine breathes and burns fuel more efficiently. That means improved MPG and reduced running costs—especially noticeable on higher-mileage engines or cars mainly used for short trips.

Restored Performance
If your engine feels sluggish or unresponsive, carbon deposits could be the reason. A carbon clean helps bring back smoother acceleration, sharper throttle response, and overall better driveability.

Prevention Is Cheaper Than the Cure
By removing harmful carbon deposits early, you reduce stress on key components like:

• EGR valve (often prone to sticking)
• Turbocharger vanes (which can jam)
• DPF (which can block prematurely)

This can save you hundreds—or even thousands—of pounds in future repairs.

Ideal Pre-MOT Treatment
Got an MOT coming up? A carbon clean is a smart way to reduce your emissions levels ahead of time and avoid last-minute fails due to high smoke, hydrocarbons, or NOx.

Is Carbon Cleaning Right for You?

It’s especially beneficial for:

• Diesel vehicles used mostly for short trips or town driving
• High-mileage engines
• Cars with sluggish performance or poor MPG
• Any vehicle close to MOT time

At MPG Tuning Ltd, we use only **safe, professional-grade equipment—no snake oil, no damage, and no shortcuts. Just proven results backed by our 19+ years of experience and 500+ genuine five-star reviews.

Based in Cannock | Trusted across Staffordshire & the Midlands  
Get in touch today to book your carbon clean or combine it with our custom ECU remap or diagnostics service.

With 19+ years of experience and over 500 genuine five-star reviews, you can count on us for honest advice and mechanically safe solutions.

Based in Cannock | Trusted Throughout Staffordshire & The Midlands  Get in touch to book your diagnostic session today.
 

IMI Certified Tuning Specialist

MPG Tuning Ltd
Unit 44 Rumerhill Business Estate
Cannock, Staffordshire, WS11 0ET

 Tel: 01543 466662
 

 

MPG Tuning Offer Professional ECU Remap and Tuning services including but not limited to ECU Remapping, EGR Delete, DPF Solutions, VAG Emissions Recall Scandal Rollback, Tricore Processor Programming, Boot Mode and Traditional Chip Tuning.

MPG Tuning is based in Cannock Staffordshire but serve customers from further afield typically from the following areas, Lichfield, Burntwood, Rugeley, Stafford, Brownhills, Walsall, Wolverhampton, Birmingham, Tamworth plus many more areas throughout the West and East Midlands.

© Copyright MPG Tuning 2018 – Monitored by OC-RiS