Often the scammer furnace guy will say that the heat exchanger is cracked, and he "had to turn off the gas supply, otherwise HE would be libel when the CO2 kills everyone in their sleep".
By leaving you with no heat, he gets you to quickly buy a new furnace.
But remember, most heat exchangers come with a 30 year (or sometimes lifetime) warranty from the manufacturer. A quick phone call will verify this. The warranty includes installation.