Field answer
Direct answer
A trap can siphon dry when venting is wrong, drainage flow pulls the seal, the fixture is rarely used, or negative pressure affects the trap arm. Use the adopted plumbing code, permit rules, and a licensed plumber for live installation or repair.
How to use this answer
Read the answer once, then prove it by choosing an answer in a practice question before looking at the explanation. If the miss repeats, tag the domain as Drainage and venting and drill a smaller set before returning to mixed practice.
Prep path
Plumbing License Study Guide
Independent journeyman and master plumber license study guide for state-specific code review, drainage, venting, traps, water supply, fixture units, and trade math.
SamplesPlumbing License Sample Questions
Public Plumbing License Practice Test sample questions with explanations.
PracticeOpen Plumbing License Practice Test
Start with the 10-question free set before deciding whether to unlock the full question bank.