Had Engine emission warning light come on occasionally for over a year. Car failed MOT on CO2 output when light was on recently. Independent garage fitted new Lambda sensor and the car then passed CO2 check. However, light came back on on way home from garage. Last year, it passed MOT when problem was happening but light wasn't on at the time. Does this point to a specific problem eg MAF/Air Mass Meter? Many thanks in advance. Passat Est 1.8T 20v Sport 2003.