Expert: Sean Halliday - 12/11/2009

Question
After MOT failed, we tried a full service, cleanout of carburettor body etc and now replaced the Lambda sensor but the emissions are still high. The garage even checked the rate of fuel flow. Now we are checking the cat. This is a 1992 'new' version Corolla - only the 9000th of its series (according to the engine number). Do you have any tips please to bring down the CO2 from about 5%?
Thanks
Chas

Answer
Possibly the engine needs a valve job because the fuel mix is way too high as the valves do not seat,causing compression ,efficiency loss.Rich fuel mix can also be the o2 sensors or the map sensor if you have one fitted but sometimes there is a maf sensor instead of the map.Too much co2 is usually worn engine components and the best solution is upper engine rebuild,because of the carbon build up.The cat will cause more higher nox readings rather than co2.Your problem is unburnt fuel.If there is a egr system on this car have that looked at as well.Check the pcv and crankcase breather system for blockage as this will cause high emissions also.