the temperature reading on my lux geo is way off, it says 75 but it feels like 68 in here.

Diagnosis

The Lux GEO WiFi Thermostat utilizes a thermistor located on the internal PCB to monitor ambient air temperature. When the reading is significantly higher than the actual room temperature (e.g., reading 75°F when it feels like 68°F), it is almost always due to "thermal ghosting." This occurs when the sensor is measuring heat trapped within the wall cavity or heat radiating from the device's own WiFi radio and backlit display, rather than the air in the room. A common culprit is the "wall hole effect." If the hole where the wires enter the wall is too large and not sealed, warm air from the attic or wall interior drafts directly onto the back of the sensor. Additionally, if the thermostat is installed near a heat-producing appliance, a lamp, or in a spot that receives direct sunlight for even 30 minutes a day, the plastic housing retains that heat, causing the sensor to report a higher temperature than the surrounding environment. You can confirm this by removing the faceplate and using a standalone digital thermometer placed 2 inches away from the wall; if the standalone device reads 68°F while the GEO continues to read 75°F, the issue is localized to the thermostat's immediate environment.

How to Fix It

To resolve this, you must eliminate external heat influences and apply a digital offset to calibrate the sensor. Follow these steps:

1. Seal the Wall Void: Gently pull the thermostat faceplate away from the wall. Inspect the hole where the 18/5 or 18/8 thermostat wiring emerges. If you see a gap, stuff a small amount of non-flammable insulation or use plumber's putty to seal the hole completely. This prevents warm wall-cavity air from blowing directly onto the sensor.
2. Check for Heat Sources: Ensure there are no lamps, televisions, or internet routers within 3 feet of the device. If the unit is in direct sunlight, install a small decorative shelf or redirect curtains to block the rays.
3. Access Temperature Calibration: Wake the screen and press the MENU button. Navigate using the arrow keys to ADVANCED SETTINGS.
4. Adjust the Temperature Offset: Locate the TEMP OFFSET (or CAL) setting. Since your unit is reading 7 degrees too high (75 vs 68), change the offset value to -7.
5. Save and Reboot: Press DONE or SAVE. Wait 30 minutes for the sensor to stabilize and verify if the reading now matches your handheld thermometer.

If That Didn't Work

• Check the C-Wire Voltage: If the thermostat is not receiving steady 24VAC power via the C-wire and is relying on "power stealing," the internal components can overheat, causing the sensor to spike. Use a multimeter to verify you have 24-28V between R and C. If voltage is fluctuating, install a 24V transformer or a C-wire adapter.
• Perform a Hard Factory Reset: A glitch in the firmware can occasionally cause the sensor to hang on a specific value. Navigate to SETTINGS > RESET > FACTORY RESET. This clears the cache and forces the thermistor to re-initialize its baseline reading.
• Verify WiFi Signal Strength: If the WiFi signal is extremely weak, the GEO's wireless chip will work overtime to maintain a connection, generating internal heat. If your signal is below -70dBm, consider a WiFi extender to reduce the load on the thermostat's internal radio.
• Replace the Backplate: If the unit was dropped during installation or the pins are bent, the sensor may be malfunctioning. Replace the wall plate (Part #GEO-WP) to ensure the faceplate is seating flush and not trapping heat against the wall.

Heads Up

• Avoid Drafts: Never install the Lux GEO directly above a supply vent or near a doorway; the rapid change in air currents will cause "short-cycling" where the AC turns on and off too quickly.
• Clean the Vents: Periodically use a can of compressed air to blow out the small vents on the side of the thermostat housing to remove dust that can trap heat.
• Calibration Limits: Be aware that the temperature offset typically has a limit (usually +/- 10 degrees). If you need more than that, the sensor is likely defective and needs replacement.
• Avoid Tight Squeezing: When mounting the faceplate, do not over-tighten the screws. Over-compressing the housing can put pressure on the PCB, occasionally affecting the thermistor's accuracy.