My house was built on a slab in1964 and has a walk out basement. There is a floor drain in my basement, along with a toilet & utility sink. Water from the main floor fixtures is backing up through the basement floor drain. I am unable to find a cleanout inside or outside. However, there is a large stack with a plug about 3' from the floor in the basement. Is this the main line cleanout? Thanks for any assistance! (https://i.stack.imgur.com/VTC1N.jpg)
Yes, it’s probably a “wall cleanout”. We like them installed above (at a higher elevation) than a tub drain, or toilet, etc. so that the cap can be removed and the drain cleaned out without sewage gushing out the cleanout.