Holy Cow! My 1994 c220 (141 KMiles) has just passed California Smog Check II with flying colors at a test-only smog station. The results are virtually perfect. I believe new ultra-low emission vehicles like Honda Civic hybrid, Toyota Prius etc. cannot even get such perfect results. Since I was required to take the car to a test-only smog station, I did quite a few preparations for it before the smog check:

- Brand new Bosch MAF (the hybrid electronic circuit of the old MAF is bad, so cleaning the thin-film element didn't help ).

- 2 brand new universal NTK oxygen sensors (the old pre-cat O2S had slow signal response and the old post-cat O2S had bad heater circuit). Total price for both new O2S is just $35! If I buy the OEM ones, it would set me back about $300.

- 4 brand new Bosch non-resistor spark plugs.

- Change new rubber connectors to make sure there's no vacuum leak. Also verify that fuel pressure regulator and the EGR work.

- Reset the HFM-sfi ECU.

- Fresh oil change.

- A 5 minutes highway run
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At 15 mph (RPM=1498):

MEAS MAX AVERAGE
-------------------------------------------------------
%CO2 15.7 N/A N/A
-------------------------------------------------------
%O2 0.0 N/A N/A
--------------------------------------------------------
HC(ppm) 1 85 21
--------------------------------------------------------
%CO 0.0 0.5 0.06
------------------------------------------------------
NO (ppm) 1 695 150
-------------------------------------------------------

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At 25 mph (RPM=1594):

MEAS MAX AVERAGE
-------------------------------------------------------
%CO2 15.7 N/A N/A
-------------------------------------------------------
%O2 0.0 N/A N/A
--------------------------------------------------------
HC(ppm) 1 50 13
--------------------------------------------------------
%CO 0.0 0.47 0.05
-------------------------------------------------------
NO (ppm) 3 717 136
-------------------------------------------------------


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Legend:

MEAS = Measured results on my 1994 c220
MAX = Maximum Allowable Emissions
AVERAGE = Average Emissions for Passing Vehicles
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Let's see who can beat my car's smog check results :-) CA DMV should exempt my car from smog check for 10 years :-)

My warmest thanks to Mr. Arthur Dalton, Mr. Steve Brotherton and Mr. M.B.DOC! I learned a quite lot about Mercedes engine management, fuel trim behavior and emission control pecularities as well as excellent ECU tricks thanks to you.

Best regards,

Eric