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Understanding The Exhaust Emissions Test

By: Kevin Watson MSc - Updated: 19 Oct 2020 | comments*Discuss
Exhaust Emissions Test Mot Carbon

Exhaust emissions are potentially dangerous. To maintain some form of control over air pollution, the European Union (EU) and the UK government have introduced an exhaust emissions test for most road vehicles. The test helps ensure that road vehicles remain properly serviced, and have relatively clean and efficient engines.

New vehicles sold in the EU must meet the standards of the test. In the UK, the test is also part of the MOT. If a vehicle has high emissions, it will fail its MOT.

The Standards

Two standards exist for UK Exhaust Emissions. The first is Euro 4, which applies to any new model of vehicle approved for sale on or after 1 January 2005. It also applies to all vehicles for sale from 1 January 2007. Euro 5 updates the Euro 4 standard. It applies to all new models of vehicles available from 1 January 2011.

The Euro 4 standards are part of EU Directive 98/69/EC. European Regulation (EC)/715/2007 is the legislation behind the Euro 5 standards.

The Emissions

The standards set limits for exhaust emissions. The regulations do allow for some variance because vehicle types, engines and testing conditions differ. The emissions the standards refer to are:

  • CO (carbon monoxide) – a toxic, flammable gas that is odourless and colourless.
  • HC (hydrocarbons) – a blend of hydrogen and carbon. Hydrocarbons form one of the main chemical components of petrol.
  • NOx (oxides of nitrogen) – occurs when oxygen reacts with nitrogen as part of the combustion process inside a vehicle’s engine. Oxides of nitrogen can be a major cause of air pollution.
  • PM (particulate mass) – minute solid particles. They may be present in the fumes a vehicle’s exhaust system emits.

Euro 4 Limits

Euro 4 limits have various categories. The three categories for cars are:
  • Up to 2.5 tonnes in weight, fully laden.
  • Heavier than 2.5 tonnes fully laden and with an unladen weight between 1,305 and 1,760 kilograms.
  • Heavier than 2.5 tonnes fully laden and with an unladen weight greater than 1,760 kilograms.

A petrol car in the first category, and with up to nine seats, has a CO limit of 1.00 gm/km (grams emitted per kilometre). The HC limit is 0.10 gm/km, and the NOx limit is 0.08 gm/km. There is no PM limit.

A similar diesel car has a CO limit of 0.50 gm/km; a NOx limit of 0.25 gm/km; a combined HC and NOx limit of 0.30 gm/km; and a PM limit of 0.025 gm/km. There is no HC limit.

Euro 5 Limits

Under the Euro 5 regulations, some of the limits, and the breakdowns of chemical categories, have changed. The Euro 5 regulations also measure the limits in milligrams emitted per kilometre (mg/km).

For a petrol car similar to the one above, the CO limit is 1,000 mg/km; the total HC limit is 100 mg/km; the non-methane HC limit is 68 mg/km; the NOx limit is 60 mg/km; and the PM limit is 5.0/4.5 depending on the vehicle.

A diesel version of the same type of car has a CO limit of 500 mg/km; a NOx limit of 180 mg/km; a combined HC and NOx limit of 230 mg/km; and a PM limit of 5.0/4.5. Diesel car engines do not have a separate total HC limit or non-methane HC limit.

Measuring Emissions

To measure emissions, a garage technician attaches a detector to the end of a vehicle’s exhaust pipe. A cable runs from the detector to a computer mounted in a portable cabinet.

The technician switches on the vehicle’s engine and runs it between 2,500 and 3,000 rpm (revolutions per minute). The computer measures the level of emissions and produces a printout. At the end of a successful MOT, the vehicle owner receives a copy of the printout signed by the technician.

Reasons for Test Failures

There are three main reasons for emissions test failures:
  • Engine damage – if components of an engine aren’t working properly, there may be excessive levels of certain chemicals in the exhaust emissions. Damaged head gaskets, pistons or cylinder rings can all result in excessive emissions.
  • A badly functioning catalytic converter – an engine’s exhaust gases enter a catalytic converter before exiting via the exhaust pipe. The catalytic converter’s role is to make the gases less harmful. If the catalytic converter isn’t doing its job properly, emission levels may be too high.
  • Faulty control system – if the emissions control system isn’t working, the car engine may not receive the right blend of air and fuel. A reduction in airflow leads to high emissions.

Regular Car Checks

To ensure that you car runs well, and has a better chance of passing its MOT, regular services are required. But you can also help by running a number of regular checks on your vehicle. Find out more by reading our article Monthly Vehicle Checks.

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[Add a Comment]
Have the emissions limits recently changed then?My car passed its last MOT with CO2 emissions of 156g/km
Epic - 19-Oct-20 @ 6:52 PM
Article is misleading. Diesels get tested on MOT for Opacity (molars -1) which is basically how smokey the exhaust is. There is no measurement of NOx or CO. PM and HC can correlate to opacity but the test is not really checking emissions. It just ensures the smoke is not so dense that it will obstruct the view of other motorists. Granted, by limiting smoke you reduce some pollutants. But you won't obtain any real values as suggested. Petrol tests give different readings. I think CO and HC are there but I don't think PM and NOx are given values. And you won't get valuable data from a 5 min test which is how long the test for the MOT is. These pollutants are tested by European labs at type testing phase and tests are run by lab techs with expensive equipment and it takes longer. If garages did that you can say bye bye to MOTs for £30. MOT is a bit of a joke anyway, so easy to manipulate a pass or fail.
BobM - 31-Oct-19 @ 10:49 AM
Having the same problem with my test results.I was hoping for details of the various pollutants, but got a similar printout.The measurements are in "microns to the minus 1" (presumably), but what exactly they are measuring is not clear.This page does not even mention the EURO 6 limits, so it is obviously out-of-date.I'd look elsewhere.
Fred - 27-Aug-18 @ 6:44 PM
I had my car tested on 5/3/2018. Although it passed, I don’t fully understand the printout, or so parts of it. It reads :- Smoke test K(1/m/)Smoke temp 14min 19max. Oil temp 81 deg c Average reading0.00 1/m Test applied1/7/2008 fast pass (limit: 1.50 1/m) Drift value 0.00 1/m
Kren - 10-Mar-18 @ 12:46 PM
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