Of all the mysteries and injustices of the McDonald’s ice cream machine, the one that Jeremy O’Sullivan insists you understand first is its secret passcode.

Press the cone icon on the screen of the Taylor C602 digital ice cream machine, he explains, then tap the buttons that show a snowflake and a milkshake to set the digits on the screen to 5, then 2, then 3, then 1. After that precise series of no fewer than 16 button presses, a menu magically unlocks. Only with this cheat code can you access the machine’s vital signs: everything from the viscosity setting for its milk and sugar ingredients to the temperature of the glycol flowing through its heating element to the meanings of its many sphinxlike error messages.

“No one at McDonald’s or Taylor will explain why there’s a secret, undisclosed menu,” O’Sullivan wrote in one of the first, cryptic text messages I received from him earlier this year.