Thermostat displays E5 and system will not energize heating or cooling stages

Safety first: Turn off power to the furnace/air handler at the breaker before opening panels.

• Step 1 – Check thermostat display: If the display is dim or cycling, suspect low voltage. Remove the thermostat and inspect R and C connections for looseness or corrosion.
• Step 2 – Measure voltage at thermostat: With power on and using caution, measure between R and C at the wall plate. You should see approximately 24 VAC. If not, proceed to the indoor unit.
• Step 3 – Inspect indoor unit board: Remove the blower door and locate the control board. Check the low‑voltage fuse (typically 3–5 A automotive style). Replace if blown, but only after checking for shorts in the thermostat cable.
• Step 4 – Check transformer output: Measure voltage at the transformer secondary. If no voltage, the transformer may be failed and must be replaced.
• Step 5 – Inspect thermostat cable: Look for any spots where the cable may be pinched or contacting sharp metal. Repair or replace damaged sections.
• Step 6 – Restore power and test: After correcting issues, restore power and confirm that E5 clears and the system responds to calls for heat/cool.