Hello, thank you in advance for any insights. Long story short: Is there ANY situation where a vent should be BEFORE (upstream) the P-trap? Long story: Our professional plumbing company, with countless customers and excellent 5 star reviews, tied into a vent, going out the roof, BEFORE (upstream) a p-trap under the shower when he redid the plumbing for our guest bathroom. (We questioned specifically why the vent was before the trap before the work was done, we even had him draw us a picture, and we were assured that this was the way it should be done in our situation.) It solved the problem of our slow drains, but since then we smell a sewage-y/earthy smell coming from the shower drain every once in awhile. We can't pin point any specific cause, it's not every time we flush for example. We had the company send a different person out to inspect it yesterday, who checked all the seals and everything else that could possibly be causing the problem. He took photos and videos of the layout and sent them to his boss. Their solution is to send the same man who installed it out to replace the P-trap with a slightly smaller one at no cost to us. Should we allow the company to replace the P-trap or is the real problem that the vent is in the wrong place and we should cut our losses and find a new plumber?