If it is indeed the test plug, it will face outboard. The bung for the sensor faces inboard, and sometimes you'll see a plug in it because the cat has the sensor installed. You won't find this plug at the Home Depot. Try an auto parts outlet.

Never mind. I just looked at your photo. The bung (part with the threads) is long gone from your pipe. By the looks of it, you should take it to a shop where it can be welded. That's the safe way to fix it short of replacement.

Oxygen sensor thread : M18 x 1.5

Test port thread : M12 x 1.5


--
Art Benstein near Baltimore

In a world without walls or fences who needs windows or gates.