Nest Thermostat F19 Fix (No Power to Y1 Cooling Wire)

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What This Error Means

F19 means “No power to the Y1 cooling wire.”

Your Nest is calling for AC, but the 24-volt signal to your outdoor unit is dead, so cooling never actually starts.

Official Fix

Do this in order. Don’t skip the safety step.

  • Kill power first. Turn off the furnace/air handler breaker and any service switch at the indoor unit.
  • Pop the Nest off the wall. Grip the display and pull straight toward you.
  • Check the Y1 wire at the Nest base. The bare copper should be straight, about 1/3 inch (8–10 mm) exposed, fully inserted, and clamped by the Y1 button.
  • Tug-test all the wires. Gently pull on Y1 and R. If a wire slides out, re-strip it and reinsert it until it locks.
  • Confirm the Y1 cable really goes to your AC. At the furnace or air handler, the same colored wire that is in Y1 at the Nest should land on the Y or Y1 screw on the control board or wire nut bundle that goes to the outdoor unit.
  • Power the system back up. Flip the breaker and service switch back on, then click the Nest display back onto the base.
  • Restart the Nest. On the thermostat go to Settings > Reset > Restart. Let it boot fully.
  • Test cooling. Set the thermostat to Cool and drop the setpoint a few degrees. If the AC starts and F19 disappears, you’re done.
  • If F19 comes back immediately. You likely have a problem in the AC control circuit (fuse, float switch, wiring, or outdoor contactor). At this point Nest’s official line is: call an HVAC pro or Nest Pro installer.

The Technician’s Trick

Here’s how a field tech narrows this down fast. Only do what you’re comfortable with around live equipment.

  • Look for a blown low-voltage fuse. Kill power again, pull the blower door, and find the control board. Most have a 3A or 5A automotive-style blade fuse. If it’s blown, replace it with the same type and rating.
  • Watch what happens. Turn power back on and call for cooling. If the fuse blows again or F19 returns, you almost certainly have a short on the Y1 circuit (wire rubbed through, crushed cable, or a shorted outdoor contactor coil).
  • Bypass the Nest to prove the thermostat. With power off, remove the thermostat wires from R and Y (or Y1) on the furnace board. Use a short jumper wire to connect R to Y on the board. Restore power. If the outdoor unit now kicks on and runs, the problem is in the Nest or its base/wiring. If nothing happens, the problem is in the AC side (float switch, wiring to the condenser, contactor, or the outdoor unit’s control board).
  • Check the condensate safety/float switch. Many systems run Y1 through a float switch at the drain pan. If that pan is full of water or the switch is bad, Y1 is cut and the Nest throws F19. Clear the drain, empty the pan, reset or replace the switch.
  • Inspect the cable to the outdoor unit. Follow the low-voltage thermostat wire from the furnace to the condenser. Look for chewed, pinched, or sun-rotted sections. Any copper touching metal can short Y1 and trigger F19.

If you’re blowing fuses, seeing burned wires, or the contactor hums but the fuse pops, stop. That’s pro territory unless you’re very comfortable with HVAC work.

Is It Worth Fixing? (The Financial Verdict)

  • ✅ Fix: System under ~10–12 years old, only a loose wire or blown 3–5A fuse, or a single bad float switch or contactor. Parts are cheap, labor is modest.
  • ⚠️ Debatable: Repeated shorts in the low-voltage wiring, water issues constantly tripping the float switch, or you need both Nest hardware and HVAC parts. Worth fixing if the system is otherwise in good shape.
  • ❌ Replace: Outdoor unit 15+ years old, compressor or control board is failing and blowing fuses, or you’ve already sunk money into multiple major repairs. Put the cash toward a modern high-efficiency system and keep the Nest.

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