All chimney dampers leak. In the winter, warm air rises out of your house and out the chimney. That warm, sheltered opening is a perfect place for birds and small critters to nest. Sometimes one unlucky sap might fall down and cannot get out. That is where they stay until they decompose. You don’t smell it because the air is being drawn out of your house.
Before you light the firebplace:
1) Go up on the roof with a flashlight and look down the flue. See anything?
2) Go back inside and open the damper, with a mirror and flashlight see if you can look up the chimney to see the sky. If it looks clear, you saved yourself $100. If not, then you need brushes, weights, etc… Not worth the effort. Call a pro and give someone some work.
I have a reburning type fireplace insert. It puts out less than a gram of particulate per hour and no smoke. (Old stoves put out 40 g/hr) The top of my chimney is covered with a cap and screen that never gets dirty and is too small let animals in. The flue pipe stays clean because I run the system hot during the cold parts of winter and don’t let it idle.
Every year I check it and only felt I had to clean it one time -mostly to satisfy myself that it really works. Fireplace design has come a long way.