Dousing the plug several times with penetrating catalyst lubricant often also called a rust breaker is another way to.
Drain clean out cap stuck.
If all your efforts fail you may have to cut out the fitting and install a new one.
Use a pipe wrench to loosen the cap on your main line drain pipe before you open it entirely.
Once it s safely removed use the old fitting as a guide to cut a replacement section of pipe and clue up a new.
Slide both couplers onto the old pipe then slide the vertical.
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Heat the cleanout plug and fitting clean away any.
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I let the water drain out then i cut out the nut using a metal cut off wheel rotozip.
Open the cap and let the buildup drain out quickly open the cap and step out of the way.