I let the water drain out then i cut out the nut using a metal cut off wheel rotozip.
Drain clean out cap stuck.
If all your efforts fail you may have to cut out the fitting and install a new one.
Slide both couplers onto the old pipe then slide the vertical.
Heat the cleanout plug and fitting clean away any.
Open the cap and let the buildup drain out quickly open the cap and step out of the way.
Make sure your reciprocating saw has a metal cutting blade before you turn it on otherwise you re in for a difficult.
Remove the cleanout plug with finesse and force photo 1.
Once it s safely removed use the old fitting as a guide to cut a replacement section of pipe and clue up a new.
Use a pipe wrench to loosen the cap on your main line drain pipe before you open it entirely.
After this i used the cut off wheel to slice away portions of the cap until i could get near the inner.