Robert, P0400 is not for the SOV, it is triggered if manifold vacuum doesn't drop enough when EGR valve is opened by the ECM. All electrical components are fine, but maybe EGR vacuum hose is bad, or the valve diaphragm leaking? Did you try to activate EGR with the hand vacuum pump? I had the EGR code in my 600SEL and cleaned everything many times. It kept coming back. Then I figured that the hole in the intake manifold was all plugged with carbon. I used a paper clip sized wire to clean it and it was tight in there. Then I found the hole is much bigger - probably close to 1/4 inch, so I did not clean all of the carbon. After I cleaned it completely - no more codes for EGR.
Mike