DIAGNOSIS Error E1 indicates the thermostat has detected a short circuit on the equipment control wires (R, W, Y, G, etc.).
FIX Turn off power to your furnace/air handler. Remove the thermostat from its wall plate. Inspect the wire connections at the wall plate for any stray strands of copper touching adjacent terminals. If clean, the short is likely at the HVAC control board. There, check for wires touching each other or a wire touching the metal chassis.
IF THAT DIDN'T WORK The short may be internal to a piece of equipment. Disconnect the low-voltage wires one at a time from the control board (Y, G, W) until the E1 error clears when you restore power to the thermostat. The last wire disconnected is the circuit with the short.
HEADS UP A common cause is the condensate overflow safety switch on high-efficiency furnaces. If the drain line is clogged, this switch can close, creating a short between R and C that triggers E1. Check and clear the drain line.