Indoor unit display shows E0 and system will not start cooling or heating

Safety first: Turn OFF power at the disconnect and breaker before removing any covers. Capacitors in the indoor unit can hold a charge; do not touch board components directly.

• Step 1 – Power reset: Turn the thermostat OFF. Switch the indoor air handler/furnace breaker OFF for 5 minutes, then back ON. Restore thermostat power and see if E0 clears.
• Step 2 – Inspect indoor PCB connections: Remove the indoor unit front panel. Locate the main control board (often behind the blower compartment on Day & Night air handlers). Verify all low-voltage harness plugs are fully seated, especially the display/keyboard connector and any EEPROM daughterboard.
• Step 3 – Check for corrosion or burn marks: Look for darkened areas, burnt smell, or swollen components on the PCB. If present, the board is likely failed and must be replaced.
• Step 4 – Verify 24V control voltage: Using a multimeter (if qualified), confirm 24 VAC between R and C at the control board. Low or no voltage can mimic an E0 fault; correct transformer or wiring issues first.
• When to call a pro: If E0 persists after a power reset and connector check, the indoor control board/EEPROM typically needs replacement and configuration by a licensed HVAC technician.