Sounds like new entries for
Ripoff Report | Scams, consumer complaints, and frauds reported.
To help these companies share the risk and give them a disincentive for just changing out parts and continuing guessing, ask them for their diagnosis in writing and the basis for that diagnosis [measurements taken and their values, instinct, product design defect, etc.].
Can you post make & model and a schematic?
My HVAC replacement age samples show an avg. replacement age of 21 to 28 years. Normal wearout is equal to this period or longer. Many people replace at 20 years, apparently with no good reason to do so.
To reduce concern about a cracked heat exchanger, 3 CO detectors will give you only 1 chance in 1 million, aircraft level reliability, of not detecting CO when it is actually present. Get the kind that detects less than 35 PPM.