I have a 96 C280 that is up for smog certification. It passed the emissions portion without any problem. The problem is there are 3 readiness monitors that will not set. They are the Catalyst Monitor, EGR System Monitor, and the 2ndary Air System Monitor. There has not been a Check Engine light since a year and a half ago when the after-cat Oxygen Sensor was replaced. Since then, the codes have not been reset or the battery disconnected. I don't know if these monitors haven't reset since that time as there has never been a reason to plug in a OBDII sensor and check them. Car has been driven many miles under various conditions including recently trying to emulate the "drive cycles" to get readiness monitors set. Any ideas as to what is preventing the monitors to set (without triggering CE codes) Bad EGR? Bad O2 sensor? Thanks.

I recently had to reset the CE codes on another C280 and all the monitors reset within a week.

Most likely the front O2 sensor, IT must be perfect for those tests to run.

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Thanks MBDoc.

Could it be the post cat O2 sensor, as well?

I saw this post which is the exact problem I have:

OBD II drive cycle to reset monitor flags

What is the "DM" that needs to be reset that Carson357 is referring to? What is the tool that is used to do the reset? I'll check with my independent MB tech if could do it then. Thanks.

On early ODB2 cars, readiness IS an issue that can difficult to fix.
So much so that MB issued this bulletin

Attached Files

Drive cycles & readiness SI 2004.pdf (105.2 KB, 13988 views)

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

I recall a '96 SL500 that wouldn't set the EGR portion of the three [Catalyst, EGR System, and Secondary Air System ] monitors, if I remember correctly. Try as they might, it wouldn't come "ready". This went on for weeks.

Finally had to remove one head (left side I think?) b/c of just enough carbon build-up in the small passages to cause this puppy to never come "ready". There were no codes!

Remind me to NEVER buy a '96 - '99 MB . .

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'94 S500 Cpe

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