I’d like to get some opinions about a call I went to the other day. The call came in as customers washing machine drain backed up. Looking at the history it shows that our company had been out in the past to snake the drain line. When I arrive I notice that the 2” drain had been re plumbed . The washing machine pumped into a stand pipe, into the trap then a T was put in with a air admittance vent. After the T was a back water valve. About 2 feet after that it connected to a 3x2 fernco which connected to a 3” cast iron Y. The washing machine drained into the back of this 3” y and the kitchen sink drained into the top. The first thing I did was disconnect the fernco and inexpert the 2” line to make sure there were no instructions . No obvious obstructions . I then looked as far down the 3” cast as I could see and it seemed to be clear aside from some normal build up. The 3” cast took a 90 at some point. After I determined the 2” was clean I reconnected it. I then removed the cold water hose from the washing machine and put it in the stand pipe. I slowly started to run water. No back ups. I then turned the valve on all the way and ran it for 5 minutes or so. I removed the cap from the back water valve and watched the water flow right on by along the bottom of the drain.No back ups. I had the customer run the kitchen sink for a extended period of time. No back ups. After that I figured it was time to run the washing machine to test it. I filled the machine and drained it. After draining for some time I heard it start to back up. It sounded like it was about to over flow from the start pipe so I shut it off. I then removed the cap from the back water valve again and filled the washing machine back up. I drained the washing machine and watched the water from the top of the back water valve. The water flowed by with no issues for some time and then I started to see it back up. I turned the machine off. After witnessing that I determined that somewhere in the 3” cast there was a partisan obstruction. I told the customer that the drain needed to be cleaned . The next day a different tech returned to clean the drain. He did some tests for himself as Well. First he filled the kitchen sink up and drained it. No issues. After the sink didn’t back up he assumed that the 3” cast was clear and that there was no sort of Blockage. He suspected that the stud or vent that vented the washing machine drain was bad. He removed the vent and filled the washing machine up then drained it. According to him it drained without any issues although he did tell Me that he could hear the water backing up but it never overflowed . When I tested it I pulled the plug before it had a chance to overflow. So he tells the customer that the drain is fine and he needs a new studor vent. I’m not to convinced that it’s the vent. The biggest reason why I feel that it isn’t the vent is that the second time I drained the washing machine I had the cap Off of the back water valve. Even with a bad vent the fact that I had the cap Off would have acted as a vent yet I could Physically see the water backing up from the back water valve. This guys story is a little twisted. He says he would hear it backing up but never overflowed. Even it he didn’t hear it back up and it drained just fine after he removed the studor vent that still wouldn’t explain why it didn’t drain properly for me when I had that back water valve cap off. My theory is either when it did back up for the customer there was another fixture putting a large volume Of water down the drain or there was a partial blockage that cleared itself. What are your Thoughts? I’m pretty certain that it will back up again sooner or later .