Why won't my display turn on, and how do I fix it?

If the display will not come on, set the system switch to OFF, remove the batteries, and insert them backward for at least five seconds to reset the thermostat, then replace the batteries correctly. You should also make sure the batteries are fresh and installed correctly, and gently clean the battery contacts using a soft pencil eraser — do not use anything abrasive on the clips. (Page 32)