E7 error and thermostat shows no power from HVAC system, may be running only on internal backup power if available

Safety: Turn off power to the furnace/air handler at the breaker before touching wiring.

• Confirm breakers: Verify the furnace/air handler breaker is ON and not tripped. Reset if necessary.
• Check R and C at thermostat: Remove the thermostat and inspect the R and C wires. Ensure they are fully inserted and clamped in the correct terminals.
• Check at furnace board: Open the furnace/air handler panel. Locate the low-voltage R and C terminals. Confirm the thermostat cable conductors are secure and not corroded or broken.
• Transformer test (advanced): With power on and using a multimeter, measure AC voltage between R and C at the furnace board. You should see ~24 VAC. If not, the transformer or internal fuse may be bad and requires a technician.
• Use C-wire adapter (if supported): If your system lacks a dedicated C wire, install a compatible TP-Link Kasa C-wire adapter kit according to its instructions.
• Replace thermostat: If 24 VAC is present and wiring is correct but E7 remains, the thermostat’s power input circuit is likely defective.