I have a 96 c280 with almost 80k. Last week, I noticed that upon startup, the air injection pump seemed to run a lot longer than it normally did. It ran for maybe 90 secs versus what seemed like maybe 15-30 secs before. This happened for a few days and then one day, after the car had already warmed up, the injection pump came on (which it normally does for maybe 5 secs every once in a while during driving) and seemed to abrubtly cut off after a shorter time, maybe 2 secs. The CEL came on at that point. The code came up with P0411. I had it reset and the CEL stayed away for about 1 week and then came back on yesterday.

During that time, the air injection pump again seemed to run the 90 secs or so upon startup, but with no other driveablility problems. I checked the vacuum hoses visually and physically where possible around the engine and under the engine covers and they looked good. I had previosly changed them about 18 mos. ago as they had cracked and caused the P0411 code before. The fan doesn't make unusual noises and sounds strong & smooth but just runs longer than before. I've read other posts that the culprits include: the aforementioned hoses, switchover valve or switch(?), EGR valve, O2 sensors, blocked EGR tube. The o2 sensors have never been changed, I've checked the EGR tube before and it has been clear (apparently they had modified the tube by this year's model?). I'm wondering about the switchover valve(s). How many are there, where are they and how to test if they are the problem. And anyone who wants to eliminate the others as the problem are welcome to. Thanks.