nest keeps switching to eco mode when i'm home, even though i have home/away assist off.

Diagnosis

The issue stems from a conflict between the thermostat's internal Passive Infrared (PIR) sensor and the "Auto-Schedule" logic, which can trigger Eco temperatures even when Home/Away Assist is globally disabled. While you may have turned off the overarching Home/Away Assist feature, the 3rd Generation Nest Learning Thermostat often retains "Auto-Schedule" patterns that it has "learned" over time. If the device believes you are typically gone at a certain hour, it may override your current state and trigger Eco mode based on a phantom schedule rather than real-time presence. Furthermore, the PIR sensor on the front of the device has a specific field of view (approximately 110 degrees). If the thermostat is installed in a hallway with low foot traffic or behind a door, the sensor may fail to detect movement. When the sensor reports "no activity" for a prolonged period, the device can default to Eco temperatures if the "Auto-Schedule" or "Eco" settings are configured to prioritize energy savings over manual overrides. You can confirm this by checking the "Care" or "History" tab in the app to see if the temperature drop aligns with a specific time of day or a lack of movement in the hallway.

How to Fix It

To stop the thermostat from autonomously switching to Eco mode, you must strip away the learned behaviors and disable the hardware sensor's influence. Follow these exact steps:

1. Disable Auto-Schedule: Press the thermostat ring to open the Quick View menu. Rotate to Settings and press to select. Scroll to Auto-Schedule and select Off. This prevents the device from creating new temperature points based on your past behavior.
2. Clear Existing Schedules: Since turning off Auto-Schedule doesn't delete old points, you must clear them. Go to Schedule from the main menu. Manually select each single temperature dot on the calendar and remove it until the schedule is completely blank.
3. Disable Hardware Presence Sensing: Go to Settings > Home/Away Assist. Select What decides if you're home. Uncheck the box for the Nest Thermostat. This tells the device to ignore its own PIR sensor and stop switching to Eco based on a lack of movement in the room.
4. Verify App-Level Location: Open the Nest app or Google Home app on your smartphone. Navigate to Settings > Home & Away. Ensure that your specific mobile device is toggled OFF for presence sensing to prevent "Geofencing" from triggering Eco mode when the app thinks you've left the perimeter.
5. Set a Manual Hold: Turn the dial to your desired temperature. If the screen says "Eco," manually rotate the dial until the background changes from a leaf/green color back to the standard orange (heating) or blue (cooling) display.

If That Didn't Work

• Check for Third-Party Integrations: If you have linked your Nest account to Amazon Alexa, Samsung SmartThings, or Apple HomeKit, these platforms can send "Away" commands that override your local Nest settings. Revoke the "Home/Away" permissions in the external app's integration settings to ensure only the thermostat is controlling the state.
• Reset the Home/Away Assist: Sometimes the cloud profile becomes corrupted. Go to Settings > Reset > Home/Away Assist. This clears the cached location data and presence patterns without wiping your entire Wi-Fi or HVAC configuration.
• Address Physical Obstructions: If you still want to use the sensor but it's failing, ensure there are no curtains, furniture, or decorative covers blocking the front face of the device. The PIR sensor cannot see through glass or plastic covers, which will trigger Eco mode even if you are standing right in front of it.
• Perform a Factory Restart: If the "Eco" ghosting persists, perform a full restart. Go to Settings > Reset > Restart. This clears the temporary memory and forces the device to re-sync its current state with the Google servers.

Heads Up

• Avoid "Eco" Overrides: Never set your "Eco" temperatures too close to your "Home" temperatures; a gap of at least 5 degrees is recommended to prevent the system from fluttering between modes.
• Battery Maintenance: If your Nest 3rd Gen is missing a "C-Wire" (Common wire), the battery may dip low, causing the device to enter a power-saving state that mimics Eco behavior. Check Settings > Technical Info > Power to ensure your Lin voltage is stable.
• Firmware Updates: Ensure your device is connected to 2.4GHz Wi-Fi. If the device loses connection, it may revert to its last known "Schedule" regardless of your current presence.
• Sensor Cleaning: Gently wipe the front plastic lens with a dry microfiber cloth. Dust buildup on the PIR sensor can reduce its sensitivity, leading to false "Away" triggers.