Assuming the horizontal discharge line is connected to the drain system inside the wall, you will need to remove the wall section to determine if that line can be lowered at the elbow or "Y"...
I BELIEVE: In a horizontal run, the p-trap must be below the vent. Otherwise, the vent may fill with water, creating a blockage, which would then syphon the water from the p-trap causing odors and...
water trap, vapor trap, P trap,,,,,,,,,,,,,,, Plummers use all those, and everyone will know what you want. You need to know if its 1 1/4 or 1 1/2 inch.
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