I have a Mercedes C180 (fuel Injection), 1995, which has just failed its emissions test. The car passed on the natural/normal idle test but failed on the fast idle test. The fast idle Lambda reading was 1.04 (the legal range is 0.97 to 1.03) and the CO level was 0.77% (the legal maximum is 0.3%). I replaced the CAT last year. The mechanic who tested the car advised that try I using a good fuel injector for 100 miles prior to a retest as this may normalise the readings. The car has been doing a number of short journeys over the past year with an occasional long trip. I will follow the advice but would welcome any other views. In particular, is it likely that the Lamda unit could be faulty. I should add that the car is running perfectly and I have no other symptoms.
Thanking you in anticipation.
John