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.