A completely clogged P-trap itself typically does not cause a sewer smell, because a blocked trap still contains standing water that acts as a physical vapor barrier blocking sewer gases from escaping into your room. However, if the blockage is accompanied by heavy organic decomposition and the water seal is partially siphoned out due to pressure fluctuations, or if the trap is dry, odors will occur. Furthermore, severe slime accumulation inside a blocked trap can produce localized foul odors. Verification: Clear branch line blockages and ensure P-traps contain fresh standing water to maintain proper odor seals.
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