Thermostat shows E4 and will not power on HVAC equipment

Safety: Turn off the furnace/air handler breaker before opening the equipment cabinet or touching low-voltage wiring.

• Check thermostat display: If the display is dim, flickering, or repeatedly rebooting, suspect a power issue.
• Inspect C wire at thermostat: Remove the thermostat from the wall plate. Confirm a wire is firmly connected to the C terminal and that the conductor is not broken.
• Inspect wiring at furnace control board: Open the blower compartment. Locate the 24V control board and verify that the same C wire is securely attached to the C terminal, and R wire to R.
• Check the low-voltage fuse: Many control boards have a 3–5A automotive-style blade fuse. If it is blown, replace it with the same rating and investigate for shorts (bare wires touching metal or each other).
• If using a C-wire adapter: Verify the Amazon-branded C-wire adapter is wired exactly as per the installation guide and that all splices are secure.
• Power cycle: Restore power and wait several minutes for the thermostat to boot and clear E4.

If E4 persists with a known-good C connection and fuse, the thermostat’s power regulation circuit may be defective and the thermostat should be replaced.