Sometimes unscrupulous furnace dealers will offer you a furnace inspection at a too-good-to-be-true low price. They may then use scare tactics to get you to buy a new furnace whether you need one or not. One method they might use in trying to sell you unneeded equipment is to tell you that your furnace has a cracked heat exchanger, is emitting carbon monoxide, must be shut down immediately, and that you need to replace it.
If they claim your furnace is unsafe, allow them to shut it down, after all, it's better to be safe than sorry. But don't sign a contract to buy a new furnace until you've gotten a second opinion, preferably from your public gas utility, as they won't try to sell you anything.
If you find you truly do need to replace your furnace, take your time do the following: 1) get several estimates from reputable companies; 2) check the companies out with your BBB; 3) hire only licensed contractors; 4) ask about guarantees and warranties on parts and labor, and get it in writing; and 5) ask for an itemized bill when the work is complete.