My house was built in 1947 (FYI - I just had the entire sewer line replaced last year). The drain pipe below my sink is now clogged somewhere past the point where it enters the foundation wall about 3 feet below ground level. There's a cleanout plug just above that point. The line is clogged somewhere beyond this cleanout and a snake won't go in more than a couple of feet. I suspect this line leads to a grease trap, even though such traps are not common in homes, since all other drain pipes/sewer lines are running just fine. Is my assumption correct or is there another possible cause of the blockage? If so, is there a simpler solution (I hope) than digging up this buried trap? NOTE: Even though the sink has a garbage disposal, it's rarely used since almost no food is disposed of this way!

IF you had a "grease trap" outside it would be a large pit with a metal cover on it. A grease trap requires servicing monthly so there is no way it would buried underground. You have some other problem, but without being there we cannot tell you what it is.

Are you on city sewer, or a septic system? Chances are that it is simply a clogged drain, but if you are on a septic, there could be other issues.

I'd contact the company that replaced the sewer line.

If you truly had a grease trap, it would have been reveal when you changed all your piping. If you had grease trap, a sewer camera could find it without digging up your yard. If you had a grease trap outdoors, you would have a exposed marker.

Best Wishes