My 01 525i with 121K has the original 02 sensors (all 4 of them ). Yesterday morning, the CEL came on. I did a full maintenance this past March (full tune up, changed fluids, other various parts, etc...). I know, my fault for not changing the sensors back then. Now since I have heard the primary sensors need to be changed between 100K-120K, I assume they triggered the CEL since I am just north of 120K. Mike Miller wrote an article about this in either Roundel or Bimmer that the two post sensors (after the cat) last up to 150K. I will order them (the primary sensors) tomorrow and get them by Friday so a friend and I can install them this weekend. They seem easy to get too. How hard would the swap be? Do I need my mechanic to clear the code after they are installed or does driving a few miles clear out the code? Any help would be appreciated. BTW, I have only driven the car at most 10 miles since the light came on. This week its parked on my driveway as I am driving my wife's 325xi. I don't want to ruin my expen$ive cat .