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Avoiding MOT Failure

Emissions - Diesel 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.

Further Help
Check Emissions
Emission Levels MOT