Thermostat displays E4 and will not allow cooling operation; cooling icon may be disabled.

Safety first: Turn off power to the furnace/air handler and outdoor unit at the breakers before working on low-voltage wiring.

Steps to troubleshoot:

• 1. Locate the outdoor temperature sensor: On many Bryant Evolution/Connex systems, the OTS is mounted near the outdoor unit or on the exterior wall, wired back to the indoor control board.
• 2. Inspect the sensor and wiring: Check for physical damage, cracked casing, or exposed wires. Ensure the sensor leads are firmly connected at the indoor control board terminals labeled for outdoor temperature.
• 3. Check for shorts or opens: With power off, gently tug each wire at the terminal to confirm it is secure. Look for splices or damaged insulation along the run and repair as needed.
• 4. Replace the sensor if suspect: If the sensor is corroded, waterlogged, or physically damaged, replace it with a Bryant-compatible outdoor temperature sensor. Land the new sensor on the same terminals and secure the wiring.
• 5. Power up and verify: Restore power and check if the thermostat now displays a reasonable outdoor temperature and the E4 code clears.
• 6. Professional diagnosis: If a new sensor does not resolve E4, the indoor control board’s sensor circuit may be faulty and should be tested by a technician.