Review on Enhanced Performance Secondary Air Injection Smog Air Pump: Mercedes Replacement - 0580000025 0001405185 by Suhas Carney

My C300 made a loud howling noise this morning when I cold started the engine. but the noise goes away after a minute or so. One day the noise stopped but the Check Engine light came on. It turned out to be an air pump. Some videos on the web suggest connecting the pump directly to the battery for testing. I would recommend against it. If the pump is stuck it will draw a lot of current and you will get a big spark. It is best to check the pump's 40A fuse and replace it if it is blown. A stuck pump will most likely blow a fuse immediately. this is what happened in my case. Turns out that air pump is one of the easiest things to replace on a Mercedes C300 - it literally took me 30 minutes - but you'll need a torx 30 bit and a few different length extensions. Also be extremely careful when loosening the bolts as you don't want them falling into the engine bay because then you'll have to jack up the car and remove the bottom plastic cover to get them back! saved what probably would have cost $500 at the dealer.