When we run the ejector sewage pump in our basement to empty it, a terrible smell comes out. It has 2 vents connected correctly. What could be causing the smell?

The ejector pump cover should have 2 pipes, 1 connected to the drain and the other a direct vent for the ejector only. Could have a leak in the seal for the piping or the lid. Can you pinpoint the place where the smell comes out of the pit?

No, we can't tell wherethe smell is coming out. It has the 2 pipes.

Pour some water around the lip of the pit, where the lid sits on the tank. Run the pump. If you see any bubbles, the seal is leaking. You can do the same thing with the piping seals. The pit should always be under negative pressure. Has this just started? Are you sure the vent pipe has not separated? The ejector system should be totally sealed. I have one for a basement bathroom and for the past 4 years it has operated flawlessly.

The Sewer Ejector vent should be dedicated vent terminating above the roof. If it is connected to the existing house vent system - then this is your problem. Milo