Thermostat displays E5 and system may not respond to heating or cooling calls; often indicates power or common wire issues.

Safety first: Turn off power to the furnace/air handler at the breaker. Low-voltage circuits can still arc if shorted.

• Step 1 – Check thermostat power: If the thermostat is battery-assisted, replace batteries. If it is hard-powered only and screen is blank or shows E5, suspect C-wire or transformer issues.
• Step 2 – Inspect C-wire connection: Remove the thermostat and verify the C wire is firmly connected to the C terminal on both the thermostat and the Carrier control board. Tighten any loose screws.
• Step 3 – Check furnace control board fuse: Open the blower compartment and locate the 3–5A automotive-style fuse on the Carrier control board. If blown, replace with the same rating. If it blows again immediately, there is a short in the low-voltage wiring that must be found before further replacement.
• Step 4 – Test transformer output (advanced): With power on and extreme caution, measure AC voltage between R and C at the control board. You should see ~24 VAC. If there is no voltage, the transformer or board is likely failed and needs replacement.
• Step 5 – Inspect for shorts: Look for bare thermostat wires touching metal or each other, especially near the furnace cabinet and outdoor unit. Repair with proper wire nuts and electrical tape or replace damaged cable.
• When to call a pro: If you are not comfortable testing live voltage or the transformer appears failed, contact a Carrier technician.