DIAGNOSIS A wiring fault or power surge has triggered a system error. The most common cause is a mismatch between the thermostat configuration and your actual HVAC equipment.
FIX Turn off power to the furnace. Pull the thermostat off its wall plate and carefully re-seat all wires. Restore power. If Err persists, you must re-run the configuration wizard. Go to Installer Setup (hold Menu button). Choose "Reconfigure Equipment." Accurately answer each question about your system (1 heat/1 cool, etc.).
IF THAT DIDN'T WORK A wire may be touching another terminal or the metal wall plate bracket, causing a short. Inspect for loose strands. Ensure the plate is not touching drywall screws.
HEADS UP If Err appears immediately after a power outage, the furnace control board may have a blown 3A or 5A fuse. Check and replace it with the same type.