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Hello, soot has built up inside the unit and probably its exhaust stack, restricting the air flow... clean all that out and you will be back to the original function.
That said, the older a heater is, the more likely its heat exchanger has cracks in it, releasing carbon monoxide into the room. Buy at least one carbon monoxide detectors, for the room the heater is in, and one for the bedrooms
Its time to locate the 'finish and rate' link and click on it, in exchange I will hold the question open for any follow up you might need.