PO411 on my E 350

Hi, I just checked the air pump, works fine Robert

Check your vacuum hoses again. There may be a leak or a hose that loosens up. Vacuum motors under the dash can do this.... They may be fine until you turn a dash knob to get air, and then the leak shows up. Turn all the air stuff off or plug the master vacuum line that goes to the dash.

Next big leak point for some is the hose going to the power brake assist.

Next is to assume the vacuum stuff is ok and the computer is lying, sort of. If there is a bad wire to any of the sensors, then the reading of the computer may be faulty.

Air Pump and Air check valves that pump air into the exhaust are indicated on many GM vehicles with this code. So make sure the Air pump and valves are working. I had one of these make a little noise and then it seized up totally!

The manifolds for your vacuum lines may also have buildup or debris in them. You can replace with OEM or custom fab with special attention to the orifices, they can be quite sensitive.

It is not uncommon for the vacuum passages to become clogged with hardened oil deposits. Check the lines attached to the manifold. There should be no build up in the metal fitting. Make sore there is a strong vacuum signal when the hose is pulled off. Good luck.