Could be the MAF (Mass Air Flow) meter - does the car run a little rough and sluggish at all??
The Lambda Sensors measure the O2 content of the exhaust to ensure the correct fueling is being used...if these are duff, it will give false information to the Engine ECU...
The MAF measures the incoming air and the ECU injects the required amount of fuel to create an even burn....the Lambdas then sense the amount of O2 coming out the other end so between the MAF and the Lambdas they tell the ECU how much fuel to inject.... if they are giving duff readings the ECU could over or unde rfuel the engine...
My first suggestion would be to get it plugged into a diagnostic computer and read the live data from the various sensors and see if anything is reading off scale or out of whack - also they could see if the ECU is storing any fault codes that could lead to an answer....
Put your loaction down in your profile - there could be a helpful member close by with the diagnostic gear to plug it in and take a gander for you....
Modern injected cars shouldn't fail emmissions, unless a sensor is reading out of whack, there is an air leak into the engine, oil is leaking into the chambers due to excessive wear, or the exhaust is shot..!!!