My washing machine, which is located in our basement, is connected directly to the main sewer line. The problem is that the main sewer line is located 5 feet up the wall and also exits the housse at that height to the city sewer line. Every once in a while, the cloths have a sewer smell. There is a valve at the connection. Could it be it is not working properly?
You never mentioned anything about a trap or vent or indirect waste. A “valve” possibly a check valve your talking about is NOT legal in most civilized countries in lieu of venting and trapping Good luck
DEMAND some money BACK from the seller so you can hire a licensed Master plumber to redo it properly. The problem also about having the washing machine below the sewer line is the very real possibility of sewerage getting into your potable water (drinking water). Get a licensed plumber to give you a price to correct this dangerous condition and then contact the last owner.
Do NOT let anyone work on this who is not licensed Good Luck and please keep me posted