Car MOT Manual 2012 onwards

7.3 Exhaust Emissions Spark Ignition

This inspection applies to all spark ignition engine vehicles with four or more wheels, except quadricycles and electric/combustion engine Hybrid vehicles.

Contained within this section are flowcharts. Carefully use these flowcharts to accurately establish which type of emission test is applicable to the vehicle being tested.

If an engine has been modified in any way, it still has to meet the exhaust emission requirements according to the age of the vehicle. A personal import must be tested according to its date of first use. The only exemption to this is if the vehicle owner can present to the tester at time of test a letter from the vehicle manufacturer stating that the particular engine as originally installed could not meet the equivalent British emission standards.

If this is the case, then test to the next lower emission standard. e.g. A 1995 car first used in Africa has a letter from the engine manufacturer stating that the particular engine engine number to be stated cannot meet Catalyst emission limits, then use the pre cat limits of CO 3.5%, HC 1200ppm. Q plated vehicles are to be treated as being first used before 1 August 1975 for emissions purposes.

The engine must be at its normal idle speed and operating temperature when checking exhaust emissions. Engine speeds and temperature can be assessed either subjectively or by reference to manufacturers or other reliable data. It is important to ensure that the gas analyser probe is inserted as fully as possible into the exhaust pipe and is secure.

Early catalyst equipped vehicles may not require a CAT test. The flowcharts and notes must be carefully followed.

To prevent the build up of fumes, the test should be carried out in a well ventilated area. Exhaust extraction systems that connect directly onto the tailpipe must not be used during the emissions test. Remote systems are acceptable, but they must not be placed closer than 250mm to the tailpipe.

Note: The availability of an extraction system is not a requirement of authorisation.
Method of Inspection
Vehicles fitted with a different engine must be tested to the requirements of whichever is older, the engine or the vehicle.

e.g. A 1995 car fitted with a 1991 engine of whatever make, test to 1991 standards for emission purposes. Note: The onus is on the vehicle presenter to prove engine age.

The following notes should be used in conjunction with the flowcharts on the following pages:

1 Two stroke engines do not require an emissions test unless they are subject to the catalyst test.

2 <= less than or equal to
3 Advice on establishing whether the design gross weight of a large car exceeds 2500kg

i it may be shown on the manufacturers VIN plate example shown in Section 3.9
ii it may be listed only in Section 2 of the current emissions data book
iii refer to any readily available data, e.g. handbook or data book iv if still unsure, assume it to be over 2500kg DGW.

A. All vehicles
1. Raise the engine speed to around 2500 rpm or half the maximum engine speed if this is lower. Hold this speed steady for 20 seconds to ensure that the inlet and exhaust system is properly purged. Allow the engine to return to idle and the emissions to stabilise:

a. assess the engine idle speed
b. assess the smoke emitted from the tailpipe at idle
c. rapidly increase the engine speed to around 2500rpm or half maximum engine speed if this is lower and assess the smoke emitted from the tailpipe. Allow the engine to return to idle.
Reason for Rejection
4 Vehicles which have been the subject of specialist conversions, are to be treated, for emissions purposes, as if they had not been converted.

e.g. a motor caravan or ambulance converted from a goods vehicle is still to be treated as not being a passenger car an ambulance converted from a passenger car or a passenger car with seats added is still to be treated as being a passenger car. A vehicle originally built with 6 or more seats, in addition to the driver, which has had seats removed is still to be treated as not being a passenger car.

5 The full title of the Department of Transport Emissions book is In-Service Exhaust Emission Standards for Road Vehicles. The latest edition must be used.

1. The engine:
a. is idling at a speed clearly above its normal idling speed
b. emits dense blue or clearly visible black smoke for a continuous period of 5 seconds at idle
c. emits excessive dense blue or clearly visible black smoke during acceleration which would obscure the view of other road users.

Note: Older vehicles, particularly pre-1960, may emit unavoidable smoke due to their design. Such smoke is not a reason for rejection.

Reproduced from the MOT Inspection Manual by kind permission of THE VEHICLE INSPECTORATE.
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