hi my motor is 99 ml320 auto does anyone know if its possible to have a bad o2 sensor without triggering the check light..car runs what i think is fine but some times the exhaust has a rich smell and sometimes feels a bit under powered but most of the time its got its power.....swapped a maf but feels same so thinking the lines of a o2 sensor...also on a 99 ml320 when you switch key and before start is the check lamp suppose to come on then go out on start......on last emission test it struggled to go through but it did just...........do not think the cats are blocked because the car does drive well when it wants to.......on the live data the long term bank 2 fuel is rich and and long term bank 1 is lean(is there a clue here)...many thanks in advance