Thermostat displays E2 and system may shut down or run erratically; often associated with outdoor unit communication loss.

Safety first: Turn off power to both indoor and outdoor units at the service disconnect and main panel. Confirm power is off before opening any panels.

• Step 1 – Power cycle: Turn off both indoor and outdoor breakers for 3–5 minutes, then restore power. Wait for the thermostat to reconnect and see if E2 clears.
• Step 2 – Inspect outdoor low-voltage wiring: Remove the access panel on the Carrier condenser/heat pump. Check the thermostat/communication wires on the control board or ABCD terminals. Tighten any loose screws and ensure correct color-to-terminal mapping.
• Step 3 – Check cable condition: Follow the low-voltage cable from the house to the outdoor unit. Look for UV damage, weed-trimmer cuts, or splices. Any damaged section should be replaced or properly re-spliced in a weatherproof junction box.
• Step 4 – Inspect outdoor control board: Look for burn marks, insect damage, or corrosion on the Carrier outdoor control board. Reseat any plug-in connectors.
• Step 5 – Continuity test (advanced): With power off and wires disconnected at both ends, test continuity of each conductor between indoor and outdoor units. Repair or replace any conductor that is open or shorted.
• When to call a pro: If wiring checks good and E2 persists, the outdoor control board or communication module may be defective and should be diagnosed by a Carrier technician.