'96 E320
* First time - replaced the popular broken vacuum feed line to switchover valve, reset P0400 code (no other codes), obvious problem - thought I was done,
* Second time - car returned, replaced EGR valve and checked EGR pipe for unobstructed flow (good flow, can hear compressed air, in intake manifold, blown into pipe w/ EGR valve removed), reset P0400 code (again no other codes).
* Before I turn it loose this time maybe I should try to do a "system" check. When energized, the switchover valve "switches over" the vacuum. But how do I test that the SOV is ever energized? What are the conditions, for the HFM M104 engine, that call for exhaust gas recirculation?
* Would self-adaptation values or a sluggish oxygen sensor ever trigger a P0400 code?
* Is there a diagnostic "flowchart" in WIS (or any other useful information for that matter) for this code in this model car? Document number?