it's calling for ac when it's cold out, or heat when it's hot. the temperature reading is way off.

Diagnosis

The Honeywell Home T6 Pro is experiencing "thermal drift" or "heat poisoning," where the internal thermistor is reading an ambient temperature significantly higher or lower than the actual room air. When the thermostat reads 80°F (due to internal heat) while the room is actually 65°F, it triggers the AC to kick in during winter, or conversely, triggers the heat during summer if it's sensing a cold draft. This is typically caused by "wall air infiltration," where unsealed holes behind the wall plate allow cold or hot attic/crawlspace air to blow directly onto the sensor. Another common cause is electronic self-heating or "ghost heating" from the wall. If the T6 Pro is mounted directly above a heat-generating appliance, or if there is a high-voltage wire running behind the low-voltage thermostat wiring, the wall cavity acts as a heat sink, radiating warmth directly into the back of the unit. You can confirm this by removing the thermostat from the wall plate and waiting 10 minutes; if the temperature reading on the screen drops significantly once the unit is away from the wall, the issue is environmental/structural rather than a sensor failure.

How to Fix It

To resolve the temperature discrepancy and stop the incorrect HVAC cycling, follow these precise steps to isolate the sensor from external influences:

1. Remove the thermostat faceplate by gently pulling it away from the wall mounting plate.
2. Inspect the hole where the wires enter the wall. If you see a gap larger than 1/4 inch, use a small piece of non-flammable insulation or a piece of plumbers' putty to seal the hole completely. This prevents "chimney effect" drafts from the wall cavity from hitting the sensor.
3. Check the wiring for proximity to high-voltage lines. If you find 120V wires running parallel to your thermostat wires within 6 inches, use electrical tape or a plastic conduit to create a barrier.
4. Reattach the faceplate, ensuring it is snapped firmly into the mounting plate to avoid air gaps.
5. Access the Advanced Setup Menu by pressing and holding the "Menu" button for 5 seconds.
6. Navigate to the "Temperature Calibration" or "Offset" setting. If the unit is consistently off by a specific number of degrees (e.g., always 3 degrees too high), adjust the offset to negative 3 to align the reading with a trusted handheld digital thermometer placed next to the unit.
7. Save the settings and exit the menu.

If That Didn't Work

• Perform a Factory Reset to clear any corrupted calibration data. Press and hold the Menu button, navigate to "Reset," and select "All Settings." This forces the thermistor to recalibrate its baseline reading from a zero state.
• Verify the "C-Wire" (Common wire) connection. If the T6 Pro is running on batteries only but is wired for 24V, internal voltage fluctuations can cause the processor to run hot, skewing the temperature reading. Ensure the blue or black C-wire is securely seated in the RC/C terminal to provide stable power.
• Check for "Micro-climates." If the thermostat is located near a kitchen, a bathroom with a shower, or directly opposite a window with sunlight hitting the casing, the T6 Pro will trigger the wrong system. Relocate the unit at least 5 feet away from any heat or cold source.
• Test for a faulty thermistor. If the temperature reading jumps erratically (e.g., moving from 70°F to 85°F in seconds), the internal sensor is likely defective. Since the T6 Pro is a sealed unit, the entire faceplate assembly must be replaced.

Heads Up

• Never use a hairdryer or heat gun to "test" the sensor, as this can permanently calibrate the thermistor to the wrong range or melt the internal plastic housing.
• Avoid installing the T6 Pro on exterior walls if possible; interior walls provide much better thermal stability and prevent the "cold wall" effect during winter.
• If you have a multi-stage system, ensure the "System Type" in the installer setup is correctly set to "Heat Pump" or "Conventional," as an incorrect system type can cause the unit to call for heat when it should be cooling.
• Periodically clear dust from the vents on the side of the thermostat casing using compressed air to ensure proper airflow to the internal sensor.