• On Arrival
• Smoke Test
• Exhaust Emission
• Under Bonnet Inspection

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On Arrival

• Your appointment card tells you which inspection lane to go to. Drive to the appropriate lane and wait.
• Present your car with the engine at normal operating temperature, this will require the engine to be kept running while waiting for the test to commence (outside test hall).
• Familiarise yourself with the vehicle controls, all light switches, boot, bonnet, fuel cap operation etc.
• Ensure any headlamp levelling device is set in the correct position for a vehicle without a load (usually a small wheel control on the dash which is numbered. In most cases the correct position will be at 0).
• Please have the vehicle registration book, appointment card and previous certificate (if applicable) readily available for examination.

On entering the inspection hall the vehicle details will be checked and entered into the computer (please have your registration book and previous test certificate if applicable ready to give to the examiner). The first test conducted will be the Smoke Test if it is a diesel car or exhaust emission test if it is a Spark Ignition (e.g. petrol) engined Car.

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Smoke Test

The smoke emitted from Compression Ignition (diesel) engined vehicles is assessed for its density, it is carried out by the use of an approved and calibrated smoke meter. The engine will be accelerated up to governed speed and the density of the smoke measured. After the 3rd acceleration the average reading is recorded. If the reading is below 2.5m-1 for non-turbocharged engines or 3.0m-1 for turbocharged engines the vehicle will pass. However if the average is higher, a further acceleration is carried out and the average of the last 3 readings are used, this will continue until a maximum of 6 accelerations have been carried out. If the average of the 4th 5th and 6th acceleration is higher than the appropriate level the vehicle will not pass the test.

In addition to the smoke meter readings any of the following will result in the vehicle being refused a certificate:

• Exhaust emits excessive smoke or vapour of any colour, to an extent likely to obscure vision.
• Emissions cannot be measured because a tail pipe is damaged or an accessory is fitted which prevents the insertion of the smoke meter probe.
• Insufficient oil in the engine or low oil pressure which could cause engine damage if engine is accelerated.
• Obvious signs of an engine defect such as an unusual noise or emission of smoke.
• Obvious signs that the governors have been tampered with or not operating.

Example computer screen showing smoke meter readings

NOTE: it is important that vehicles are properly maintained (including changing of timing belts in accordance with the manufacturers' recommendation) and presented for test at normal working temperature.

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Exhaust Emission

This test applies to all cars, taxis, minibuses and ambulances with up to 12 passenger seats, and other vehicles up to and including 3500 kg design gross weight (DGW), which are spark ignition engined with four or more wheels. Any of the following points will result in the vehicle being refused a certificate:

• The engine is idling at a speed clearly above its normal idling speed.
• The engine emits dense blue or clearly visible black smoke for a continuous period of 5 seconds at idle.
• The engine emits dense blue or clearly visible black smoke during acceleration, which would obscure the view of other road users.
• The emissions cannot be measured because the tailpipe is damaged or an accessory is fitted which prevents insertion of the analyser probe.
• The exhaust gas contains one or more gases in excess of the required limits for the associated test.
  
  The required limits are as follows:
  • Vehicles first used prior to 1 August 1975 are exempt from a metered emission test.
  • Vehicles first used on or after 1 August 1975 and before 1 August 1986:
     
    - Carbon monoxide (CO) 4.5% at idle
     
    - Hydrocarbons (HC) 1200ppm (parts per million)
    
    
  • Vehicles first used on or after 1 August 1986 and before 1 August 1992:
     
    - Carbon monoxide (CO) 3.5% at idle
     
    - Hydrocarbons (HC) 1200ppm
    
    
  • Vehicles first used on or after 1 August 1992 are tested to the manufacturers specific limits (there are a few exceptions to this rule depending on the fuel and vehicle type). While specific limits are too numerous to list the following is for guidance only:
     
    - Hydrocarbons (HC) 200ppm
     
    - Carbon monoxide (CO) 0.5% at idle
     
    - Carbon monoxide (CO) 0.3% at fast idle
     
    - Lambda: 0.97 - 1.03 Lambda (value of 1 for the optimum air/fuel ratio setting)

NOTE: It is important to remember the above figures can only be used as guidance as vehicle specific limits may be lower or higher.

Vehicles must be presented for test with the engine at normal working temperature. Therefore engine should be kept running on arrival at the test centre.

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Under Bonnet Inspection

An under bonnet inspection is carried out at this stage (please be ready to release the bonnet catch when requested).

Also checked at this stage is:

• Registration plates and vehicle identification number (VIN)
• Power assisted steering
• Engine transmission mounts
• Mechanical brake components
• Hydraulic, air and vacuum brake systems
• Fuel system
• General condition of the vehicle
• Vehicle structural integrity and construction