Spider nets or webs blocking the air tube as mentioned above ... the air tube usually has either slit openings or an opening where the gas and air mix together before the burner itself.
Compressed air to blow everything out (with the gas off) generally cleans it out well.
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2009 Salem PT 403FB
2005 Ford XLT Supercrew
Roger the above post! Something is dirty and the LP isn't being burned completely, and is emitting carbon monoxide. The average person can fix it. Google it, or check here RV Water Heater Woes