Cooling is a chain of relays. The Nest closes its internal switch on a call for cool. That signal travels on the Y wire to the AC's contactor. The contactor pulls in and energizes the outdoor compressor and condenser fan. Anything that breaks the chain — a loose Y wire, a wrong setting in the Equipment menu, a tripped contactor, a low-pressure cutout from low refrigerant — looks the same to the user: thermostat says cooling, house stays warm. Working the chain in order finds the cause without guessing.

Quick checks before opening anything

  • Is the thermostat set to Cool (not Heat or Off)?
  • Is the setpoint below the room temperature?
  • Is the air filter clogged? A dirty filter triggers freeze protection and looks like "not cooling."
  • Are both the indoor and outdoor breakers on?
  • Is the outdoor unit running (fan spinning, compressor humming)?
  • Has cooling worked recently, or did this start at install?

If the outdoor unit isn't running

  1. Check the outdoor disconnect (a pull-out switch near the condenser) — it should be in.
  2. Check the dedicated outdoor breaker in the main panel. Reset once if tripped. If it trips again, stop and call.
  3. Open the Nest app or device and look for any "Delayed" or "Starts in" message — that means the Nest is waiting on the compressor lockout timer (normal up to about 5 minutes) or sensing low power.
  4. If the Nest battery icon shows it's low, the thermostat doesn't have enough steady power to close the Y relay. This is the C-wire story below.

The C-wire story

Nest learns to "power-share" off the heating and cooling wires on simpler systems, but cooling-only setups, heat pumps, and many modern variable-speed systems need a dedicated common (C) wire to keep the device steady. Symptoms of an undersized power supply: the thermostat reboots randomly, the WiFi drops, the cooling delay message shows up repeatedly, the compressor cycles short. Google now recommends a C-wire for almost every install and sells the Nest Power Connector as an alternative. Open the system air handler or furnace cabinet (breaker off first) and trace which wires are landed at the C terminal. If a wire is at C inside the equipment but not at the Nest, run that lead through the existing wire bundle and land it on the Nest's C connector.

If the equipment type is wrong (especially heat pumps)

Nest asks during setup whether the system is conventional (separate furnace + AC) or a heat pump. On a heat pump, the O/B wire energizes the reversing valve for cooling (or heating, depending on which side). If the equipment type is set to conventional on a heat pump install, the Nest won't drive O/B and the system stays in heating mode even on a cool call.

  1. On the Nest, go to Settings → Equipment.
  2. Confirm system type matches what's installed. For a heat pump, choose Heat pump and verify O or B is configured per the manufacturer (most US heat pumps are O; older ones are B).
  3. If the equipment type was wrong, change it, return to home, and call for cool. The outdoor unit should engage within a minute.

If the Nest calls for cool but the AC stays off

  1. Confirm the Y wire is landed on the Nest's Y1 terminal. The button on the Nest connector should click in and hold the wire.
  2. Take the Nest off its base. Use a multimeter to confirm 24V AC between R and C. No voltage = the system isn't powering the thermostat.
  3. Inside the air handler/furnace cabinet (breaker off), check whether the Y wire is landed at the equipment side and is intact.
  4. Test the contactor: with the system call active, the contactor at the outdoor unit should pull in audibly. If not, the call isn't reaching the contactor (broken Y wire) or the contactor has failed.

If the outdoor unit runs but cold air doesn't reach vents

  • Replace the air filter first. A clogged filter ices the indoor coil within an hour.
  • Shut the system off for 2 to 3 hours to let any ice on the coil melt before retesting.
  • Confirm the indoor blower is running. If the fan is set to Auto, it should run when the AC is calling.
  • If the outdoor unit runs but the vents blow warm or weak, that often points to low refrigerant charge — licensed HVAC work.

When to call an HVAC pro

  • Outdoor breaker that trips immediately on reset.
  • Outdoor unit runs but vents stay warm after a filter change and coil thaw.
  • Ice visible on the refrigerant line set or indoor coil even after a long off cycle.
  • Burning smell, oily residue near the outdoor unit, or any visible leak.
  • Repeated low-power warnings on the Nest even with a confirmed C-wire connection.
  • Configuration changes on the Equipment menu that don't make the system respond.

Good maintenance rhythm

  • Monthly during cooling season: check and replace the air filter as needed.
  • Quarterly: rinse the outdoor condenser coil (breaker off) and clear leaves and grass from the surrounding area.
  • Yearly: have a licensed HVAC tech check refrigerant charge, electrical connections, and the indoor coil drain.
  • Yearly: confirm the Nest's Equipment settings still match the installed system after any HVAC service.
  • Twice a year: check that the C-wire termination at the Nest is firmly seated (gently pull on the wire — it should hold).
  • Ongoing: log any "Delayed" or "No power" messages from the Nest; patterns point to power issues.
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