At our previous house there was an interesting tactic used by Furnace companies that I must tell folks about.
Our house, at the time, was part of a larger development where most of the houses
were built by the same builder and about the same time. The Furnace Repair companies knew what kind of furnace was installed in most of the houses, and knew of a fault in the design of these furnaces which caused the Aluminum heat exchanger to crack prematurely (i.e. but still long after any warranty on the furnace).
The furnace companies had a neat trick, where they would offer folks a Cheap Furnace Cleaning and a Free Furnace inspection. Inevitably the free inspection was done first and if the furnace did have a cracked heat exchanger it was “red tagged” (i.e. turned off and not allowed to be turned on again until repaired or replaced) and then the chap who did the inspection would then say that his company couldn’t repair the problem and the furnace would need to be replaced. As a matter of coincidence this same company sold furnaces as well, and you can guess how the story might end.
Was it dishonest ? Well I fell for it, and ended up having to get a new furnace, thanks to this fun bit of “salesmanship”.