Changed at 55K...

My mechanic hooked up the reader and checked the response time. He checked at 45k and noticed they were getting slow. Finally at 55k, the check engine light came on and they were replaced. Fairly starightforward DIY job on the C280. Took me about 1 hour. Each one cost about $90. Get the OEM sensor, the universal ones you need to splice the connectors, not a good thing to do.