I recently had a reader email this question. I sent a short answer, but thought this was deserving of a longer answer in the form of a blog post.
In reading this email, I see three assertions that need to be addressed:
- HVAC businesses are using cracked heat exchangers to scam people.
- HVAC businesses in Minnesota lock out furnaces when cracked heat exchangers are found.
- Carbon monoxide detectors will keep people safe from carbon monoxide.
Cracked Heat Exchanger: Sales Scam?
I checked out the COmyths web site. There's a lot of good info on there, but some of it is misleading and there are a lot of straw man arguments made. For example, the headline at the beginning says "Myth #1 - A furnace with a cracked heat exchanger will definitely produce carbon monoxide and poses an immediate danger. (Wrong!)" . Yes, that's an incorrect statement, but I've never actually heard anyone say that. That headline is easily made true by rewording the sentence just slightly: a cracked heat exchanger has the potential to increase carbon monoxide levels, and has the potential to pose a danger to the occupants.
So what's a cracked heat exchanger all about and what's the big deal? I wrote a blog post many years ago discussing that topic, and the gist of my blog post was the same as what's stated on the COmyths web site; a cracked heat exchanger probably isn't as dangerous as many folks make it out to be, but the furnace (or heat exchanger) still needs replacement. Here's the post: How Serious Is A Cracked Heat Exchanger?
If a heating contractor finds a cracked heat exchanger and says the furnace needs to be replaced, they're not pulling a scam unless they're just outright lying to you about finding a crack. They're simply doing their job. Towards the bottom of the page on the COmyths web site, you'll find this text:

See? This isn't a scam, even according to their web site.
Do HVAC businesses lock out furnaces?
Sometimes. If a company considers the heating equipment to present an imminent danger to the occupants, they might disable it, but this policy varies from company to company.
If a heating company unscrupulously disabled a furnace in an attempt to strong-arm the sale of a new furnace, I would take serious issue with that. I don't believe that happens though. At least I hope it doesn't.