Outdoor unit intermittently shuts down, L2 or low-voltage fault indicated, thermostat may go blank

Safety first: Turn off power to the indoor unit before accessing the control board or transformer. Low voltage can still cause shorts and damage components.

Steps:

• 1. Check thermostat display: If blank, remove it from the wall plate and inspect for corrosion or loose terminals.
• 2. Inspect low-voltage fuse: On the indoor control board, locate the 3–5A automotive-style fuse. Replace if blown. If it blows again, there is a short in the 24V circuit.
• 3. Look for wire damage: Trace the thermostat cable and condenser control wires. Pay special attention where they pass through sheet metal or conduit. Repair any nicks or crushed sections.
• 4. Test transformer output: With power on and using a multimeter, measure between the transformer’s secondary leads or R and C on the board. You should see ~24 VAC. If not, the transformer is likely failed.
• 5. Secure connections: Tighten all R, C, Y, G, and O/B terminal screws at the thermostat, indoor board, and outdoor unit.
• 6. If L2 remains unresolved: Further diagnosis with a wiring diagram is needed; contact an HVAC technician.