Diesel Emission MOT Failure

Normally the diesel emissions is rarely an issue, so if your car has failed it's MOT on this recently, it maybe came as a surprise. The diesel MOT emission test limits were changed mid 2018. We've had two in today that failed their MOT, the smoke emitting criteria is a 3rd of what it was last year! 

So what to do... unfortunately there is no magic answer, there can be many issues that could cause high emissions. We'd start by either adding a fuel treatment and a good road test or carrying out a diagnostic test, in which we'd check;

  • Diagnostic / computer test to check for any fault codes
  • Check through the live data from the sensors fitted to your car
  • Check for any known issues
  • Intake leak test to check for external air leaks

Assuming the above is ok and the fuel treatment had no effect, the most common fix for the emission issue is an induction service.


Car servicing in basic consists of changing the engine oil to keep the engine internals clean, fuel filters to keep the fuel system clean and air filters to keep the air going into the engine clean. However nothing is done to the induction system, this is where the air flows before reaching the internals of the engine. 

The induction systems often become clogged with oil & carbon deposits. This can sometimes be detected via the above diagnostic tests, depending on how bad the situation is. 

We offer an induction cleaning service, which will break down the carbon and grime that's inside your induction / engine. 

The reason that a build up of this carbon can cause emissions to fail, is that it restricts the amount of air flowing into the combustion chamber (the important bit inside the engine) - the lack of air means that the diesel being injected doesn't burn efficiently, resulting in a failed emissions test. 

After this induction service, your engine will require an oil and filter change. This is because some of the chemicals that are used to break the carbon down, will mix with the engine oil. 

So if you're in Lincoln and you're diesel has failed it's emissions, give us a call to discuss these options. 

What does it cost?

This depends on the vehicle but is normally around £300 plus the diagnostics and oil change. 

What does it look like?

This is what the carbon buildup can look like inside your engine. This restrics the amount of air that can flow through the engine. 

Our induction service will help to release and remove this carbon.

In extreme circumstances, the only way to correctly repair is to strip the induction system and manually clean. 

Diesel Emission Limits?

What are the limits?

Most newer cars have a manufacturer plate, with the vehicle identification number. At the bottom of this plate the manufacturer lists the emission limit tolerance.

If there is no plate, then the limits are dependent on the vehicle age;

Vehicle first used Type Value

1. First used before July 2008 Non-turbo 2.5m-1 or plate value if lower
Turbo 3.0m-1 or plate value if lower

2. First used on or after 1 July 2008 All diesels 1.5m-1 or plate value if lower

3. First used on or after 1 January 2014 All diesels 0.7m-1 or plate value if lower