Gather the current behavior safely
Use the normal owner controls to confirm Heat mode, a target above the room temperature, and whether a schedule or hold is changing the request. Record the display's exact error, room temperature, and whether the furnace begins any part of its normal cycle. Take a short recording only from a safe position with the panels closed. Do not recreate an alarming event to get a better recording.
Honeywell Home's heating troubleshooting distinguishes thermostat settings, system type, and whether the equipment responds. Its page is a guide for supported controls, not a universal wiring recipe. Honeywell Home heating checks.
Do not move wires, force stages, change installer settings, or bypass a protective control. Tell the technician about any changes made since the first test, including a new sensor, schedule, accessory, or repair.
Ask what the old test covered
Did it confirm only that a heat request began, or did it run and check the applicable heating functions? Was it a thermostat-only check or a furnace inspection too? Ask the provider to distinguish what was verified then from what is verified now.
Thermostat power trouble can also appear after installation. Google lists later power symptoms in its Nest common-wire guidance. That is one possibility to check if your product fits, not evidence that a C wire is automatically the answer. Nest post-installation power issues.
Address the cold home while arranging repair
Tell the service provider how fast the indoor temperature is falling and whether anyone has difficulty tolerating cold. Ask what can be checked promptly and when service is actually available. If the house cannot stay suitably warm while you wait, consider staying somewhere warm rather than relying on a thermostat reset to solve it.
Your next step is a coordinated callback that verifies the thermostat, the furnace's response, and the final heating result. Keep the symptom record open until the system responds as intended under the applicable operating conditions.