Avoiding MOT Failure

Emissions - Petrol Engines

Make sure that the engine is in good working order and tuned. Ignition timing, clean air filter etc.

Start the engine and hold the revs at around 2500 rpm for about 20 seconds.

Allow the engine to return to idle and watch for any smoke from the exhaust pipe.

If dark smoke comes from the exhaust then the car will fail and need engine repair or further tuning.
Blue smoke usually means oil is being burnt. This usually occurs when worn seals allow oil into the cylinders. (Valve stem oil seals or piston rings.)
Black smoke usually indicates unburnt fuel. This usually occurs with faults in the injection or carburettion system or a blocked or dirty Air Filter.

Check that the idle speed is not too high (Usually between 600 - 1000 rpm)

For any further tests you will need the use of a gas analyser capable of measuring Carbon Monoxide and Hydrocarbons. These can be hired or you could take your car to a garage for a check.
Levels permitted for each car vary by model but the amount of hydrocarbons and level of Carbon Monoxide produced from your exhaust must not exceed the levels set by the manufacturers for model and age of car.


Vehicles to be tested

In-use exhaust emissions testing is applied to all petrol and gas-powered vehicles with four or more wheels.
The test does not apply to vehicles fitted with 2-stroke engines.
The following types of vehicle will be considered as first used before 1 August 1975

(Visual Test):
• Wankel rotary engined vehicles first used before 1 August 1987.
• All kit-cars and amateur built vehicles first used before 1 August 1998.
1.1 Types of test
The emissions test to which a vehicle is subject will depend upon its date of first use (i.e. date of registration or date of manufacture if used abroad before first registration in the UK) as follows:
• For vehicles first used before 1 August 1975 a visual test will be applied.
• For vehicles first used on or after 1 August 1975 a metered test will be applied.
On 1 January 1996 a new test was introduced for petrol fuelled passenger cars fitted with advanced emissions control systems such as three way catalytic converters. The test was extended to include large petrol fuelled passenger cars and petrol fuelled light goods vehicles from 1 August 1997.

The test applies to:
• Passenger cars first used on or after 1 August 1992 and mentioned in the Annex to this publication.
• All passenger cars first used on or after 1 August 1995.
• Other vehicles, such as large passenger cars and light goods vehicles, first used on or after 1 August 1994 and mentioned in the Annex to this publication.
For all petrol engined vehicles first used before 1 August 1992, all gas engined vehicles and vehicles other than passenger cars first used before 1 August 1994 the test procedure consists of a metered check at the normal idle speed, the so-called `non-catalyst. test.

Some passenger cars first used during the "transition" period (i.e. 1 August 1992 to 1 August 1995) and some light goods vehicles and large passenger cars first used on or after 1 August 1994 were not required to be manufactured to the EC standards requiring advanced emissions control systems to be fitted. For these vehicles the non-catalyst test will be applied. Owners of vehicles which were first used during this transition period, are advised to check the entries in the Annex to this booklet or contact the vehicle manufacturer.

Fuel & Exhaust
Emissions - Diesel
Catalytic Converters
Diesel Particulate Filters

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