How Emissions Sensors Help Your Car Pass the MOT

Modern vehicles are marvels of engineering, designed to be more efficient, cleaner, and safer than ever before. But as any UK driver knows, that progress comes with responsibility — particularly when it comes to MOT testing. One of the most common (and costly) reasons cars fail their MOT is due to emissions problems. And at the heart of emissions control are a group of hardworking but often overlooked components: emissions sensors.

In this blog, we’ll break down exactly what emissions sensors are, how they work, which ones matter most, and how they help (or hurt) your chances of passing your MOT. Whether you're a curious driver or someone who's recently had an unexpected MOT fail, this article will give you the clarity you need.

?️ Why Emissions Matter for Your MOT

Since 2018, MOT rules in the UK have tightened considerably — particularly around emissions. Vehicles are now more likely to fail if there’s an illuminated engine management light (EML), or if onboard diagnostics (via the OBD port) detect problems with emissions-related components.

If your car’s exhaust emissions are too high, or a sensor responsible for monitoring emissions is faulty, the DVSA requires garages to record it as a major defect — which means an immediate MOT failure.

In petrol cars, excessive CO or HC levels can be the problem. In diesels, smoke opacity and DPF (diesel particulate filter) function are key. In both cases, emissions sensors are what trigger the warnings, log the data, and keep the car compliant.

? Emissions Sensors Explained (and Why They Matter)

Let’s explore the most important emissions sensors found in modern vehicles, what they do, how they work, and how a fault could land you in MOT trouble.

1. Oxygen (Lambda) Sensor

Often located in the exhaust manifold, the lambda sensor measures the amount of oxygen in the exhaust gases. Its job is to help the engine adjust the air/fuel mixture for optimum combustion and emissions.

• If it fails: The engine may run rich (too much fuel) or lean (too little fuel), which increases CO emissions and decreases fuel economy. It may also trigger the EML.

• In an MOT: An incorrect air/fuel ratio can cause your car to fail the emissions test.

? What is the OBD Port and Why Should You Care?

2. NOx Sensor (Diesel & Petrol Direct Injection)

NOx (nitrogen oxide) sensors detect the amount of harmful nitrogen compounds in the exhaust — gases known to worsen air quality and affect health.

• If it fails: The vehicle may inject too much fuel or fail to trigger AdBlue systems, increasing pollution and harming the selective catalytic reduction (SCR) process.

• In an MOT: While not always directly tested, a faulty NOx sensor can cause increased smoke or emission-related fault codes that fail the test.

3. DPF Pressure Sensor (Diesel)

This sensor measures the pressure difference before and after the diesel particulate filter. It helps the ECU determine if the filter is becoming clogged.

• If it fails: The DPF may not regenerate properly, causing soot build-up, reduced performance, and eventually limp mode.

• In an MOT: Any warning lights related to the DPF or excessive diesel smoke are considered a major fault.

? Can My Car Fail Its MOT for Warning Lights?

4. Exhaust Gas Temperature Sensor (EGT)

This sensor monitors the temperature of the exhaust gases and helps regulate turbocharger function, DPF regeneration cycles, and emissions.

• If it fails: The ECU might delay or cancel DPF regeneration. It may also cause poor engine performance or overheating of the turbo.

• In an MOT: May result in increased particulate emissions and poor test results.

5. Mass Air Flow Sensor (MAF)

Although not technically in the exhaust stream, the MAF sensor measures the amount of air entering the engine. This information is critical for fuelling and emission control.

• If it fails: The ECU can’t calculate fuelling accurately, which leads to over-fuelling (increased CO emissions) or under-fuelling (performance loss).

• In an MOT: Emissions might rise beyond legal limits even if no warning light appears immediately.

6. Crankshaft & Camshaft Sensors

These sensors monitor engine speed and position, allowing the ECU to adjust fuel injection timing and ignition.

• If they fail: You might experience rough idling, misfires, or non-start conditions — all of which can impact emissions.

• In an MOT: If a misfire code is logged or fuel isn’t combusted efficiently, the car could fail.

⚠️ Warning Signs You Shouldn’t Ignore

Most emissions sensor failures will cause the engine management light to appear. But there are other signs too:

• Increased fuel consumption

• Strong smell from the exhaust

• Black or blue smoke

• Sluggish performance

Even without a light, you might still have pending fault codes — only visible during a diagnostic scan.

? Book a diagnostic scan at Brede Garage — we use SNAP-ON Solus Legend tools to get it right first time.

? How We Fix It at Brede Garage

Our approach to emissions sensor faults includes:

• Full OBD-II scan using manufacturer-level diagnostics

• Verification of sensor function via live data

• Physical inspection of exhaust components

• Repairs or replacements using OE-quality parts

We don’t just “clear codes” — we identify root causes and fix them properly.

? Summary: What Every Driver Should Know

• Emissions sensors are your car’s way of self-monitoring pollution and performance

• If they fail, your MOT is at risk — even if the car seems to run fine

• Warning lights = immediate attention

• Regular diagnostics help avoid surprise fails

? Book your MOT or diagnostics with Brede Garage and drive with confidence — clean, compliant and fully sensor-smart.