There could be a number of reasons why your car has emissions that are too high to pass the NCT. The first is simply a lack of use, especially if the car is only used for short journeys, so the first thing to do is give it a good run, keeping the revs high though the gears, as this should clear a lot of residual hydrocarbons from the exhaust system. If not, then it could be the catalytic converter at fault, or a failing lambda sensor, also known as the oxygen sensor, or a sticky Mass Airflow (MAF) sensor. If a code reader shows up no faults in the emissions system then the MAF is the most likely culprit and can often be fixed by cleaning it up with carburettor cleaning spray. It's also worth putting a fuel system cleaner into the petrol tank and giving the car a run, as this can often bring the emissions down. The catalytic converter is the most expensive fix here, so we'd rule out the other possible problems first.