Old tricks that help passing MOT emissions test :
1) AND MOST IMPORTANT: Make sure the engine is REALLY WARMED UP. For about half an hour prior to test, flog the car like there's no tomorrow. Use 3/4 of throttle pedal max, not WOT, to keep AFR at stoich (= max EGT = hottest CAT), alternating acceleration bursts up to 56K rpm in 3rd gear with off-throttle coastings. If after this you have to spend some time in the queue, never stop the engine, in fact giving it a bit of throttle to keep it idling @ 2000 rpm will help. For emissions, the warmer the engine the better.
2) Fresh air filter will help.
3) Fresh engine oil will help. On high mileage cars with worn rings/valve stem seals, thicker oil will help.
4) Fresh spark plugs of correct type will help.
5) For emissions, 95RON is better than 98 or higher RON gasoline.
Last ditch tricks:
A) Remove air filter just prior to MOT, then place back right after MOT.
B) Block all PCV lines just prior to MOT, then unblock right after MOT.
C) Replace spark plugs with one or two grades 'hotter' ones prior to MOT, then switch back to standard heat range plugs right after MOT.
