Simple Heating Cooling & Refrigeration

I had a minor emergency over the past weekend and called a couple of the 24/7 providers because the company we normally use was not available. The first one diagnosed a part needed but he didn't have one, said he could come back on Monday. The second one, who told me on the phone that he had the part, started out by saying the first guy was an idiot, then said that the problem wasn't that part, but said my 'heat exchanger' was cracked! And that since the furnace was over 15 years old I might as well replace it. And while I was at it, I should replace the A/C as well, because, he said, the coils may begin to leak after changing out the furnace. And, he warned, that 'cracked heat exchanger' could put out carbon monoxide! Talk about scare tactics! (The only good thing I can say about this guy is that he did not charge us for the service call since he didn't get anything done. Looking back, I think that might've been a ploy to gain our trust so we would believe his story and spend $3500 with him to replace out system.) So we put up with the cold for the weekend and waited to get a second opinion from AC Service Co, the people we normally use but were unavailable last weekend. First thing they did, to put my mind at ease, was check for a crack in the heat exchanger. There was no crack. They determined a part was missing, a simple 'flame sensor', and had the heat back on in less than an hour. I will never trust any other heating & air company besides them again!