HVAC guys help please!

Hi. I have a problem with a condesing gas style propane furnace..It seem to heat ok, but the exhaust coming out the vent has a foul exhaust smell, never smelled like this before..I can see blue flames inside the unit, so I assume the burn is ok...Any ideas what I am in for?
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This is Turtle.
When you have a foul oder in the vent gases. Your not burning all the gas and waisting fuel. The vent gas can have a little off smelling oder in it but not foul oder. You can look at the blue flames in there but you will also see some white somewhere to get the foul oder smell.
It needs cleaning , adjusting, or something to stop this unburnt fuel. Unburnt fuel addes to the cost to operate it.
TURTLE
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