placo1, when i lived in the san fransico area,walnut creek, i did 5-10 mercedes per day. about a third to half failed the the test which then prompted the machine to do a three minuite warm up at 2,500 rpm . the way the test is set up i don't get tail pipe numbers till after the test is finished. yes,these cars were shut off first till i went to fetch them in the lot. most were there only a short time. i let them idle for 5-10 minites, then did the test.there were a lot of 2-4 year old cars with low miles. any way even though i had to pre condition them because they failed the first part of the test, at the end they passed with flying colors.3-5 hc's and almost no co.
also the same car will have differant tail pipe readings if tested at differant parts of the day.
since your car passed without being preconditioned the cat could have been not as hot as it should be. hence the 02 that was still there
duke i am still learning ,i like your answers
good luck rich

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88 300E White, sold 9-04
88 190E Black 2.3 sold
90 300CE White, Grey sold 12-24-04
92 400E White,Grey Sold 5-06
90 300CE 91K Signal Red,Black Sold 2007
1989 300CE Smoke silver,Tan Sold 1-08
1993 300E 3.2 M104 motor
1991 300CE White,Grey
1994 E420 beautiful car,gone to new home
1994 E320 Smoke an Parchment.Beautiful car