Emission Readiness Monitors
 

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.

Thanks MBDoc.

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

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

http://www.peachparts.com/shopforum/tech-help/57960-obd-ii-drive-cycle-reset-monitor-flags.html

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

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 . . :(

Not sure if this is relevant. I hear the secondary air pump run for the 90 seconds or so at cold startup. I do not ever notice it running again after the car has run and then comes to a stop (not engine stop but a stop at a light , for example after a freeway run.) I think that's when it runs a test of secondary? I seem to recall this occurring in the past but no longer. Is this possibly an issue?

That's normal . . should run for 2 minutes or so at cold startup. The purpose is to inject "air" into a very rich exhaust. When the car warms, it's not needed again.

Thank you Jim,

So the running of the second time after engine has already warmed up is not always going to happen, ie, if it ran ok at cold startup and checked out?

Jim,

I see on your website the diagram of the relationship between the DM (N59) and the ECU, where it shows that the DM is responsible for the ON signal for both the secondary air and EGR diagnostic. This is my problem. EGR and Secondary air not achieving readiness. I know the DM is functioning as I put it another C280 and these 2 monitors ran and went to ready. My ECU, via a Snap-On MT2500 can be commanded to run the secondary air fan and EGR valve. Possiblity of the wiring from the DM to ECU being problematic? Or simply not communicating. Anyone know the strategy of testing the connection or communication between the DM and ECU. I'm going to guess that the O2 sensor/heater and purge monitor don't need a command from the DM to ECU, thus they achieve readiness (the only ones that do)?

Thanks for any input.

Nice site by the way, Jim.

The secondary air diagnostic, as I've mentioned before, that should run when in closed loop, has not been running. In the past, I have noticed this running after achieving closed loop, ie, when the engine is warm. Running at cold startup is fine.

I'm no expert but I'd bet you have the same problem that the '96 SL500 had . . . some carbon buildup in the egr passages. . . hope not but if it hasn't come 'ready' by now, then it could be head-off time. :(