Susan, the "90" series of thermostats covers a couple of different thermostats made by White-Rodgers. Here's how to find your model number
. With that info, I can check the manual specific to your thermostat.
In the mean time, check the display for the word BATTERY or BAT. (Flashing) on the display. Battery flashing or blinking on
the digital display indicates low batteries. If the batteries have
recently been changed and the thermostat continues to indicate low
batteries, use a pencil eraser to polish each of the battery contacts
inside the back of the thermostat. Do not bend them up and down when
polishing or you might break them. When complete, they should all be as
shiny as a mirror. This will assure proper battery back-up, allow the
backlight to work correctly and remove the flashing battery symbol from
the screen. Battery (Solid) on Display The battery symbol (not blinking)
indicates a loss of power (24 volts) from the heating and cooling
equipment to the thermostat. Normally the heating and cooling do not
operate when the battery indicator is on. If the battery indicator is
on, check the fuses or breakers, make sure the furnace is switched on,
and verify the blower compartment door is closed properly. The battery
display is designed to indicate the thermostat is running on batteries,
so replacing batteries has no effect on the indicator. See No Heat/No
Cool for more information. Note: Some heat only systems (zoned systems)
disconnect power from the thermostat during a normal heating cycle and
display the battery symbol. Millivolt heating systems always indicate
battery because they do not supply 24 volts to the thermostat.
A beep often is provided as audible feedback when keys are depressed on many thermostats. You should check to make sure that no key is stuck in the "in" position or is otherwise activated.