Thermostat shows E8 and HVAC shuts down shortly after starting

Safety: Turn off power to both furnace/air handler and outdoor unit at the breaker before inspecting wiring or equipment.

• Check for rapid cycling: Observe whether the system is turning on and off every few minutes. This can trigger protective lockouts.
• Inspect low-voltage wiring: Look for staples driven too tightly, crushed cable, or bare copper touching metal around the air handler and near the thermostat.
• Check filter and airflow: A severely clogged filter can cause overheating and short cycling. Replace the filter and ensure vents are open.
• Reset furnace/heat pump: Many furnaces have a power switch on the side. Turn it off for 5 minutes, then back on to clear lockouts.

If E8 returns and the equipment is short cycling, have a professional check compressor and blower motor current draw. If wiring and equipment test normal, the thermostat’s internal protection circuit may be faulty and the thermostat should be replaced.