Thermostat displays E5 and system will not run or runs erratically

Safety first: Turn off power to the furnace/air handler at the breaker. Low-voltage circuits can still be damaged by shorts even if they are not dangerous to touch.

Steps to troubleshoot:

• 1. Check display behavior: If the Emerson thermostat screen is dim, flickering, or rebooting, suspect power issues.
• 2. Inspect thermostat wiring: Remove the thermostat from the wall plate. Verify the R (or Rc/Rh) and C wires are firmly clamped in the correct terminals on the sub-base. Tug gently to ensure they are secure.
• 3. Look for shorts: Ensure no bare copper from any conductor is touching another terminal or the mounting screws. Trim and re-strip wires if necessary.
• 4. Check furnace control board fuse: At the furnace/air handler, remove the access panel (power off). Locate the low-voltage 3A or 5A blade fuse on the control board. If blown, replace with the same rating and inspect for shorts before restoring power.
• 5. Confirm presence of C wire: Emerson smart thermostats require a common (C) wire or a compatible power adapter. If C is missing or loose, install or repair it according to Emerson’s wiring guide.
• 6. Restore power and test: Turn breakers back on and see if E5 clears. If the error persists, the thermostat’s power input circuitry may be damaged.

When to call a professional: If fuses continue to blow or you cannot identify a short, contact an HVAC technician to test the transformer and low-voltage wiring.