I have recently began trying to repair a DIY washing machine box/standpipe issue. I tried connecting my washing machine drain hose into the existing washing machine box/standpipe. As soon as the washing machine went to drain, the standpipe on the washing machine box was not able to handle the volume and over flowed quickly. After doing some troubleshooting I opened up a section of the wall and found this. I found that the standpipe was reduced from 2" to 1 1/2" and piped into a common drain with the sink. I also found that a P trap was never installed allowing and sewer gases through. In addition to that, the A/C condensate line was also piped into the same standpipe.
So basically I have a lot of work in front of me. Can anyone give me any insight into why the drain is overflowing when the washing machine drains? Is it because the standpipe is 1 1/2" or is it because the drain does not have a P trap or a vent? I have done enough plumping projects around the house but I am a little lost on where to start with this one. Any help would be greatly appreciated.