Thermostat shows P002 and reports low power; display may dim or shut off when calling for heat or cool.

Safety warning: Turn off power to HVAC equipment at the breaker before opening panels or moving wires.

• Step 1 – Check wiring connections: Remove GLAS and inspect the R and C terminals for tight, clean connections. At the furnace board, tighten the R and C screws and ensure wires are not corroded.
• Step 2 – Measure voltage under load: With a multimeter, measure between R and C at the GLAS wall plate while the thermostat is calling for heat or cool. Voltage should remain near 24 VAC. If it drops significantly (e.g., below 18 VAC), the transformer may be undersized or failing.
• Step 3 – Remove extra loads: If humidifiers, UV lights, or other accessories are powered from the same 24 VAC circuit, temporarily disconnect them to see if P002 clears. Overloading the transformer can cause low voltage.
• Step 4 – Inspect cable length and gauge: Very long thermostat runs with thin wire can cause voltage drop. If the run is unusually long, consider upgrading to a heavier gauge cable.
• Step 5 – Replace transformer or board: If voltage is low at the control board itself, the 24 VAC transformer or control board is likely failing and should be replaced by a technician.
• Step 6 – Suspect GLAS hardware: If voltage is stable at the wall plate but P002 persists and the thermostat browns out, the internal power regulator on the GLAS main board may be defective, requiring thermostat replacement.