When the "Repl Batt" message came up we replaced the batteries according to instructions and the thermostat continues to show the error message. It will look almost normal for a minute: it displays the current temperature and even says the system is set to cool, but in the upper left corner there is a message saying "Wait" and the "Repl Batt" error blinks... until a minute later when everything turns off except the battery replacement message. No buttons pressed will stop this from happening.

No pins are bent, the thermostat was running fine before we left the house for the weekend, and we've tried several brand-new batteries. The terminals are clean, we've reset the circuit breakers, nothing. We've even tried a reset hack with putting in new batteries upside-down to cut the power completely and drain the capacitors. Any ideas?

Sorry I forgot to mention the model! A 5-year-old Honeywell Chronotherm IV Plus