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I have an in-wall laundry drain - pictured below.

Since we moved in it had a significant clog which I unclogged with a combination of wet-vac sucking on the drain and snaking the vent pipe. Finally we got it to drain without flooding but now there's an awful sewer smell after doing laundry. Before unclogging, while it did flood fairly quickly, there was no sewer smell

So my questions are:

  • How can I determine if the sewer smell is from a lack of p-trap in the wall (ideally without removing drywall)?
  • Could the sewer smell come from another clog?
  • Whether or not there is a p-trap in the wall, could I just put a p-trap outside of the wall attached to the plate to solve the smell?

in wall drain

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  • Take a picture with flash down the pipe and you might be able to see whether there is a water surface (indicating presence of a trap) or a dry pipe. (There could be a bend before a trap, though.) – Kevin Reid Nov 6 '20 at 18:29
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    have someone flush a toilet in the house ... listen at the laundry drain ... if you hear the rush of water, then there is an open path to the sewer – jsotola Nov 6 '20 at 18:34
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    drain the washing machine into a bucket and smell it – jsotola Nov 6 '20 at 19:27
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    look for a floor drain ... the vacuum cleaner may have sucked water out of the trap in the floor ... it may be refilled by running water at a tub ( thin tube from tap to drain) – jsotola Nov 6 '20 at 19:33
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    @JeffWheeler great tip thank you! I lowered a rope and about 19 inches past the floor the tip got very wet. Woohoo! – byte Nov 6 '20 at 20:28
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How can I determine if the sewer smell is from a lack of p-trap in the wall (ideally without removing drywall)?

It would be very unlikely for there not to be a trap; they are required by code. More likely you have a trap that is being siphoned dry, which can be caused by various problems including poor venting, clogged or partially clogged drain, poor design.

Could the sewer smell come from another clog?

YES... well not really coming from another clog, but caused by a clog or partial clog.

could I just put a p-trap outside of the wall attached to the plate to solve the smell?

NO you can't piggy-back traps. If there is a trap in the wall (and I would bet $5 that there is) you can't install an additional trap.

I would recommend that you ensure that all house drains are running free and clear (by whatever means necessary) and same with all vents. The fact that you merely wet-vacced the line and did not snake the drain line (

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  • I couldn't get my 1/2" snake past a certain point on the drain line. When I wet-vacced and snaked the vent I got a ton of stuff out of there. Should I get a thin augur to snake the drain line? Do you have recommendation on how I should ensure the vent is free and clear? – byte Nov 6 '20 at 20:02
  • Based on @jeffwheeler's comment above, there is a p-trap since lowering a rope into the drain caused the tip to get wet. So you're probably correct there is a p-trap. So I guess I have a partial clog that I need to get figured out. – byte Nov 6 '20 at 20:33
  • The stuff down in the vent pipe is often debris that carried up into the vent and lodged there due to a drain line clog. Most definitely you should snake out the drain line. – Jimmy Fix-it Nov 6 '20 at 21:26
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I had an issue with the smell as well. In my case, I had to make sure that the washing machine's drain tube was not inserted too far down into the wall drain. After reading a suggestion on the web, I retracted some of the tube back out of the drain. The smell went away after the p-trap resealed itself with water. The smell hasn't returned (3+ years ago).

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