Customer washing machine was overflowing whenever running a load, upon arrival, I tested and saw that the washing machine was backing up whenever we did a load. Next, I gave an option to run my drain cable down to remove any buildup. After running my drain cable, I removed a lot of buildup. After testing, I was able to get the washing machine flowing back normally. I told the customer that they could have cast-iron and if it is cast-iron, it could be rusting on the inside but underneath the sink with ABS so I wasn’t 100% sure but after testing everything thoroughly the drain is now keeping up..