There are two solenoids provided with one source. The two outputs go to the EGR and the secondary air cut-off valve.
The vacuum is switched at the times that each system functions which is totally different for each system. The sec air comes on cold and stays on for some period of time...no other control. The EGR does not come on cold, or at idle, or at full throttle. It is open all the warm running time in the lower partial load area and some into upper partial. So switching the output lines causes the systems to work i a way that the EGR is first to be faulted on as it is a continuos monitor. Secondary air is only monitored one time per drive cycle and then only if certain conditions exist.
If you look closely in the photo one can see a big black handled screw driver levering the pipe away from the hole. That is all that is necessary to get at the plug.
Bosch Master, ASE Master, L1
33 years MB technician