Originally Posted by pizzaguy

The P4010 indicates a problem with the secondary air injection pump at the front of the engine - this pump runs to inject extra air into the exhaust to preheat the CATS faster. It is usually caused by broken solder connections on the Relay Control Board. In 70,000 miles, mine has done it twice. I yank the RCM and reflow solder around the (obvious to the eye) broken joints at the connector on the board and all is well for about two years, then it does it again. Happens to a lot of us.

P0456 is indication of a leak in the evaporative emissions control system (that is, the gas tank pressure test that the car does every few engine start/stop cycles.) This is most often caused by a loose or defective gas cap.

I believe the part you are referring to is a check valve used in the EGR system. But 401 and 456 are not EGR codes. I THINK my new EGR valve came with one of those already installed, but I can't say for sure and I'm 770 miles from my car. The EGR is on the passenger side of the engine in the back - remove the air intake and it's staring at you at the back of the cylinder block, atop the valve covers. There is NO mention of this part in the emissions control chapter (25) of the service manual and I notice that it is not listed as a part for the SLK320 at all. Why did you buy that part?