What I've tried so far

I have this P-trap that I thought was standard. I bought a transparent replacement described as "Fits 1-1/2 in. or 1-1/4 in. ABS" but the fitting (reducing gasket) was too small. So I'm guessing that this is a non-standard size. In my searches I haven't been able to find a larger size that's transparent.

What I'm trying to do

I believe that I might have a siphon problem. In order to narrow down the problem I want to replace that P-trap with a transparent one so that I can see what's happening.

Looking for a solution that would help me get a transparent P-trap fitted onto this non-standard size coupling or some other solution that would help me understand if I have a siphon or other problem.

Problem is that it smells like sewer gas is coming back in. I've already removed and cleaned out this P-trap and replaced it once before but it doesn't make a difference and the P-trap was pretty clean and non-smelly itself.

I've read this answer and that might be what I need but that question seems to be a different issue.


  • 4
    Either clean out the clogged vent, or add a Studor (AAV, Mechanical vent) if this thing is set up without a vent (should be going up from where the sanitary tee enters the wall) Fooling around with a clear trap is a waste of time, as you clearly have a venting issue due to the smells you report. >99% of the time it's a clogged vent, <1% of the time the inspector who passed the work should be vilified for not doing their job. Or the work was never inspected (that might be more than 1%...)
    – Ecnerwal
    Jan 27, 2023 at 2:10
  • If your P-trap is glued at the back, then it will have only one kind of adjustment--it can swing in an arc. This is not really ideal because it is hard to line up with the vertical tube (called the tail piece) draining the sink. When it is glued like this it must be very carefully fitted before it is glued or it will leak. You really want a slip joint at the back allowing in-and-out translation, and rotation about that axis. This gives three different types of adjustment to get perfect alignment. Jan 27, 2023 at 7:58
  • Your P-trap is not "standard", it has a union joint instead of the usual ones used with tubular drain fittings. I'm guessing the threads are 2" IPS. I would recomment cutting it out and glueing a trap adapter to the pipe emerging from the wall. In my experience, plastic union joints, unless they are the sort that uses a rubber seal, are nothing but trouble.
    – kreemoweet
    Jan 27, 2023 at 19:07

2 Answers 2


Your likely problem is indeed improper venting.

You ask about ways to diagnose and confirm the problem.

When you have a foul smell, drain the P trap and measure the amount of waste water removed. You can catch it in a small container to see how much was in the trap.

If you don't like opening the unions of the one you have (it can be tricky and stiff at times), a transparent P trap would work of course, but also a "P Trap with Cleanout", a.k.a. a "P Trap with Drain Plug".

enter image description here

Image: Rona / Ipex.

If you have replaced the trap with the capped one, pour the water you drain from the new one in the old P trap to give you a quick idea of whether the trap is open or closed.

An AAV (Air Admittance Valve, a.k.a. cheater valve) under the sink will help, but you need to check your plumbing to determine the root cause. Your current dry vent may be clogged, and it may not be properly venting several other drains as well.

An AAV is not a substitute for circumventing a clogged vent.

They are allowed to assist venting a branch of your drain, but cannot be used in lieu of venting to the exterior. Any back pressure or methane build-up from the sewage will not be able to vent away through an AAV, and dangerous sewer gases can push through the P trap with sufficient pressure. Vents avoid this by relieving the back pressure.

Your vent stacks are likely visible on the roof, and you should inspect them to see if anything is stuck, like a pine cone, a clog of leaves or a rodent.

enter image description here

  • 2
    "see if anything is stuck, like a... rodent." Ew. Plumbers always seem expensive to me until it comes down things like stuck rodents. Here, take my money. How much did you say? Here, have some more. Jan 27, 2023 at 19:00
  • @WayneConrad true, it's always the ugly dirty tedious and especially unglamerous part of a profession that really set the fees....
    – P2000
    Jan 27, 2023 at 19:09
  • 1
    Makes you wonder why they don't put a simple bit of screen on the top of these roof pipes, to prevent critters or other stuff from getting in there in the first place. It would add pennies to the cost, a huge bargain compared to what you end up paying someone to clean it out. Jan 27, 2023 at 19:38
  • 1
    @TimSparkles yes. I just find removing the cap a lot easier. OP doesn't have to replace anything, but I assumed what was there was not suitable for the diagnostic task. I'll edit to clarify.
    – P2000
    Jan 27, 2023 at 21:55
  • 2
    @DarrelHoffman I think the biggest issue is ice that forms when warm humid sewage air mixes with freezing outside air at the screen or in a U neck
    – P2000
    Jan 27, 2023 at 22:22

As mentioned, you need to vent it properly, and retrofitting is easier with an Automatic Air Vent (AAV).

Just to show you what it may look like (I am not associated with it).


Grabbed from Waste Plumbing: Air Admittance Valves and Automatic Air Vents

  • The AAV should be located as high as possible, which that one is not. And it should screw in, which that ones does, as they fail and need to be replaced. But first, clean the vent, if (as is likely) there is one already.
    – Ecnerwal
    Jan 27, 2023 at 13:28
  • 6
    AIUI, "as high as possible" is, at a minimum above the flood level of the device being vented. i.e. above the rim of the sink in this pic.
    – FreeMan
    Jan 27, 2023 at 13:33
  • I despise the things, so I'm not much up on the details, but I think someone mentioned a minimum of 4" above the trap weir; and it's not uncommon fro them to be located in the cabinet, where they can't possibly be above the flood level on the sink on the cabinet - though it is better to get them higher, such as running them up in the wall cavity to an access panel. Less frequency of failure that way. If in the cabinet, as close as possible to the underside of the counter (just enough room to unscrew the bad one and screw in a new one) is best for that case.
    – Ecnerwal
    Jan 27, 2023 at 14:56
  • 1
    @Ecnerwal: Many kitchen islands have a bulgy thing near the sink which contains an AAV inlet.
    – supercat
    Jan 27, 2023 at 19:51
  • Dishwasher air gap is the common one above the sink, IME.
    – Ecnerwal
    Jan 27, 2023 at 20:19

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