8 Reasons Cars Fail MOT on Emissions: Causes, Fixes & Prevention
MOT emissions failures are one of the most common — and most frustrating — reasons a vehicle doesn’t pass its test. Whether it’s high CO, excessive smoke, a faulty lambda reading or a blocked DPF, the root cause is almost always the same: combustion inefficiency.
This guide breaks down every major emissions category, explains why failures happen, and shows you the most effective ways to fix and prevent them — without unnecessary repairs.
How MOT Emissions Testing Works
Petrol Vehicles
- CO (Carbon Monoxide)
- HC (Hydrocarbons)
- Lambda (Air–Fuel Ratio)
Diesel Vehicles
- Smoke opacity
- DPF presence & performance
- Visible smoke (automatic fail)
Official DVSA emissions standards:
GOV.UK MOT Inspection Manual – Emissions
1. High CO (Carbon Monoxide)
What High CO Means: The engine is running too rich — too much fuel, not enough air.
Common Causes
- Dirty injectors
- Worn spark plugs
- Faulty oxygen sensor
- Weak ignition system
- Carbon buildup in the combustion chamber
Fast Fixes
- Fuel system cleaner
- Engine flush
- Spark plug replacement
- Check for vacuum leaks
Recommended XADO Solutions
- XADO Verylube Petrol Injector Cleaner
- XADO Atomex Petrol Restore
- XADO Engine Flush Soft
2. High HC (Hydrocarbons)
What High HC Means: Unburnt fuel is leaving the exhaust — incomplete combustion.
Common Causes
- Misfires
- Worn piston rings
- Poor compression
- Dirty injectors
- Faulty ignition components
Fast Fixes
- Injector cleaning
- Compression restoration additives
- Ignition system check
Recommended XADO Solutions
- XADO 1 Stage Revitalizant
- XADO Verylube Injector Cleaner
3. High NOx
What High NOx Means: The engine is running too hot or too lean.
Common Causes
- Carbon buildup
- EGR valve sticking
- Faulty lambda sensor
- Weak cooling system
Fast Fixes
- EGR cleaning
- Engine decarbonisation
- Cooling system check
Recommended XADO Solutions
- XADO Anti-Carbon Cleaner
- XADO Atomex Complex Oil Treatment
4. Lambda Failures
What Lambda Measures: The air–fuel ratio. A failed lambda reading means the mixture is incorrect.
Common Causes
- Faulty O2 sensor
- Exhaust leaks
- Dirty MAF sensor
- Fuel pressure issues
Fast Fixes
- Replace O2 sensor
- Clean MAF
- Check for exhaust leaks
Recommended XADO Solutions
- XADO Verylube Petrol System Cleaner
5. Diesel Smoke Failures
What Smoke Failures Mean: Your diesel engine is producing excessive soot.
Common Causes
- Blocked injectors
- Worn turbo seals
- Low compression
- Poor-quality fuel
- Blocked air filter
Fast Fixes
- Diesel injector cleaner
- Turbo cleaning
- Engine flush
Recommended XADO Solutions
- XADO Verylube Diesel Injector Cleaner
- XADO Atomex Diesel Restore
- XADO Engine Flush Soft
6. DPF-Related MOT Failures
Automatic Fail Conditions
- DPF removed or tampered with
- Visible smoke
- Excessive soot load
Common Causes
- Short journeys
- Low exhaust temperatures
- Blocked DPF
- Faulty sensors
Fast Fixes
- DPF cleaner
- Forced regeneration
- Long motorway run
Recommended XADO Solutions
- XADO DPF Cleaner
- XADO Diesel Restore
7. How to Prevent MOT Emissions Failures
- Use a fuel system cleaner every 3,000–5,000 miles
- Flush the engine before oil changes
- Use high-quality fuel
- Take a 20–30 minute motorway run before MOT
- Maintain the air intake system
- Use revitalizants to restore compression
8. When You Should Worry About a Bigger Problem
If you still fail after cleaning and preparation, the issue may be mechanical:
- Low compression
- Worn piston rings
- Faulty catalytic converter
- Turbo wear
- EGR failure
At this point, diagnostics are essential — but many vehicles pass after proper cleaning and preparation.
Conclusion
The majority of MOT emissions failures come down to carbon buildup, dirty injectors, poor combustion, or DPF issues — all of which can be fixed or prevented with the right maintenance routine.
Your XADO product range gives drivers a way to restore compression, clean injectors, reduce smoke, improve combustion and protect the engine long-term.