Thermostat displays E12 and shows a low-voltage or power supply error; screen may dim or reboot intermittently.

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

Steps to troubleshoot:

• 1. Verify C-wire presence: Remove the thermostat and confirm that a wire is connected to the C terminal on the wall plate and at the indoor control board. If no C-wire is present, a dedicated C-wire or Bryant-approved power accessory is required.
• 2. Tighten connections: At the indoor unit, locate the C terminal on the control board and ensure the thermostat’s C conductor is firmly attached. Tighten any loose screws.
• 3. Inspect cable: Check the thermostat cable for damage, splices, or signs of overheating. Replace damaged cable with new 18/5 thermostat wire.
• 4. Check low-voltage fuse: Inspect the indoor control board’s low-voltage fuse and replace if blown, after correcting any shorts.
• 5. Test voltage: With power restored, a technician can measure between R and C at the thermostat to confirm 24 VAC. If proper voltage is present but E12 persists, the thermostat’s internal power circuit may be faulty.
• 6. Replace thermostat if needed: If all wiring and board checks pass, replace the Bryant smart thermostat.