If your Nest thermostat isn’t cooling or heating, start with the settings: make sure the mode is set to Cool, Heat or Heat·Cool, that Eco mode isn’t active, and that no schedule is overriding your temperature. Then look for a “Delayed” message or a low battery warning, which usually means the Nest isn’t getting enough power and needs a C wire. If the Nest is calling for heat or cooling but nothing happens, the problem is likely at the furnace, air handler or outdoor unit, not the thermostat.
Step 1: Check modes and schedules
- Mode: in the Nest app or on the device, confirm it’s set to Cool, Heat or Heat·Cool, not Off.
- Eco mode: Eco temperatures (often 62°F heat / 82°F cool) can make it seem like the system isn’t responding. Turn Eco off.
- Schedule: a scheduled setpoint may override your manual change at the next time block.
- Home/Away Assist: if the Nest thinks no one is home, it may switch to Eco.
- Temperature lock or safety temperatures set by someone else.
Step 2: Look for a “Delayed” message
Nest thermostats can delay heating or cooling for a few minutes to protect the compressor or to recharge their internal battery. A “Delayed” or “In 10 minutes” message is normal occasionally. If it appears constantly, the Nest probably isn’t getting enough power. See Nest “Delayed” message.
Step 3: Check power and battery
- In the app or Settings > Technical Info > Power, check the battery voltage. A low or dropping battery usually means power stealing isn’t enough.
- A C wire usually solves power problems. See how to install a smart thermostat without a C wire.
- Make sure the furnace door is closed and the breaker is on. A Nest that goes dark points to a power problem; see thermostat blank screen.
Step 4: Check the wiring and equipment settings
- Under Settings > Equipment, confirm the Nest detects the wires you actually have (W1, Y1, G, C, O/B).
- Heat pumps: a wrong O/B setting makes the Nest heat when it should cool and vice versa. See thermostat wiring colors explained.
- After a recent install, re-check that each wire is fully pushed into its connector.
Step 5: Run a quick test
In the Nest’s settings, use the equipment test (if available on your model) or simply set a temperature well above or below room temperature and listen at the furnace or outdoor unit. If you can hear the system click but nothing runs, or nothing happens at all, the thermostat is doing its job and the HVAC equipment needs attention.
If the thermostat is calling but the system won’t run
- No cooling: see AC not cooling and why is my AC blowing warm air.
- No heat: see furnace not turning on or heat pump blowing cold air.
Older Nest models
Google ended app support for the first- and second-generation Nest Learning Thermostats in October 2025. Those units still work as basic thermostats on the device itself but can’t be controlled remotely. If you rely on app control, consider upgrading.
Battery warnings or the display shutting off? See Nest thermostat low battery.
Heating works but bills are high? See Nest thermostat using aux heat too much.
Frequently asked questions
Why is my Nest thermostat not turning on the AC?
Common reasons are Eco mode, a schedule, a “Delayed” message from low power, a wrong O/B setting on a heat pump, or a problem with the AC itself.
How do I reset my Nest thermostat?
Restart it from Settings > Reset > Restart. A factory reset erases schedules and settings; use it only as a last resort.
Does Nest need a C wire?
Not on every system, but many power-related problems disappear once a C wire is added.
Disclaimer: this guide is for general information only. Turn off power at the breaker before touching thermostat wiring. Contact a licensed HVAC professional for electrical diagnosis.