E420 idling too fast

Hello: I just replaced the throttle valve assembly on a '94 E420. The reason I replaced it is I had a check engine light that came on. I cleared the code and I would get an idle speed control code which would recur after about three days. After replacing the throttle valve assembly, the car would idle at 1500 RPM. I removed the new throttle valve assembly, checked for misplaced hoses and gaskets, found none so I reinstalled the original throttle valve assembly. At this point the car idled normally. I suspect a defective part.

Does anyone have any thoughts on this? Do I have a bad brand new throttle valve assembly??

Any help is greatly appreciated!

Thanks,
Mark