In fact, Kytch made an aftermarket add-on to the machines to let the restaurants know what is wrong, without having to call Taylor for service.

McDonald's corporate came out and said they weren't allowed to do it, that it was dangerous to do it instead of calling Taylor, that it risked leaking McDonald's proprietary business information.

In fact, Taylor makes ice cream machines for a number of fast food chains, but McDonald's explicitly got the crappiest one (not as in "cheap", but explicitly crappy, as McDonald's corporate mandated their use).

Now why in the world does Taylor make good machines for other chains, and why in the world were operators not free to buy the better models or even fix the models they are allowed to buy? Taylor's business results talk up the revenue for their paid repair service, and with the operators having easy access to know how to fix it/avoid the problem themselves, their repair volumes would go down. Evidently they have a particularly interesting business relationship and thus McDonald's corporate was happy to through franchisees under the bus in service of that relationship.

Nowadays, it seems franchisees are allowed to get machines that actually self-report the problems. Presumably all the bad press around the whole ordeal moved it to the point of really needing to be fixed (also two vendors are allowed now instead of just Taylor)

There's lots of ways that you could solve the problem. It's not designed to be solved.

Or just a line stamped into the metal reservoir tub

You could even put an etch mark in the tank if you were the tech to fix the problem after the fact.