When the battery is removed and the pin pulled up to the 3v bus, the thermostat works flawlessly. I traced the circuit, it goes between ground and the uC pin,

When the battery is removed and the pin pulled up to the 3v bus, the thermostat works flawlessly. I traced the circuit, it goes between ground and the uC pin, through a 1 Mohm resistor, no other connection. It’s about right for killing a 300mah battery over 10 years. The pin is held low when the unit is energized by the thermostat signal lines.