Nest Thermostat F16 Fix: Fast Error Code Guide

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What This Error Means

F16 on a Nest thermostat means the thermostat sees too much electrical current on one of the low-voltage control wires, usually the call for heat or cool (W or Y).

Translation: the Nest thinks there is a short or an overloaded circuit, so it shuts that output off and your furnace or AC will not run.

Functionally, you treat F16 the same way as Nest’s overcurrent faults (like E16/N16): find the bad wire or device that is pulling too much power, fix it, then restore power.

Official Fix

What the manual and Nest support expect you to do:

  • Kill power first.
    Turn off the furnace or air handler breaker, and the outdoor AC breaker if you have one. Do not pull wires live.
  • Pop the Nest off the wall.
    Gently pull the thermostat head straight off the base. You are looking at the low-voltage wires.
  • Check for obvious shorts.
    • Any bare copper touching another terminal? That will trip F16.
    • Stray strands of wire frayed out and touching the ring or another wire? Trim and re-strip clean.
    • Two wires shoved into one terminal that should not be there? Fix that.
  • Verify wiring against Nest’s diagram.
    • Open the Nest app or install guide for your exact model.
    • Make sure each wire is on the right terminal: R/Rh/Rc, W/W1, Y/Y1, G, C, etc.
    • If you see a wire pushed into the wrong spot, move it where it belongs.
  • Reseat every wire.
    • One at a time, press the tab, pull the wire out, straighten and re-strip to about 6 mm (1/4 inch), and firmly reinsert.
    • Give each wire a light tug. If it slips, it was not clamped and can arc or short.
  • Focus on the call the system was using.
    If F16 hits while cooling, suspect Y/Y1 and C. If it hits on heat, suspect W/W1 and C. Check those wires extra hard for damage or mis-wiring.
  • Inspect at the furnace or air handler.
    • With power still off, remove the blower door panel.
    • Find where the thermostat cable lands on the control board (R, C, W, Y, G terminals).
    • Look for crushed, chewed, or melted thermostat cable between the furnace and wall.
    • Any nicks or bare copper touching metal? Cut back and re-terminate, or replace that run.
  • Look at the low-voltage devices.
    • Cooling: check the 24 V wires going out to the outdoor unit contactor (usually Y and C).
    • Heating: check wires going to gas valve, zone valves, or boiler relays.
    • Any splices in the line should be tight and taped, not twisted and exposed.
  • Restore power and test.
    • Put the furnace panel back on so the safety switch is pressed.
    • Turn breakers back on.
    • Snap the Nest back on its base.
    • Call for heat or cool again and see if F16 clears.
  • If F16 keeps coming back:
    • Remove the suspect call wire (Y or W) from the Nest and cap it temporarily.
    • If F16 goes away with that wire removed, the problem is in that circuit (wiring or the device), not the thermostat itself.
    • At this point the official line is: call a qualified HVAC technician to find and fix the shorted device or wiring.

The Technician’s Trick

Here is how a field tech usually chases an F16 faster than the manual.

  • Bypass the Nest to split blame.
    • Power off the furnace.
    • At the control board, pull the thermostat wires off R, W, Y, G.
    • Use a small jumper wire and manually jump R to W (heat) or R to Y (cool).
    • Power back on briefly. If the system runs fine with jumpers and the low-voltage transformer does not buzz or get hot, the Nest or its base is suspect.
    • If it blows the low-voltage fuse or the transformer groans, the problem is in the equipment or field wiring, not the thermostat.
  • Use a meter instead of guessing.
    • With power off, check resistance between R and the suspect call (usually Y or W) at the control board.
    • A dead short (close to 0 ohms) means crushed wiring or a shorted contactor/valve coil somewhere down the line.
    • Disconnect the field wires at the outdoor unit or valve and recheck resistance to see which side is bad: cable or device.
  • Take the load off the Nest.
    • Older contactors, zone valves, or add-on humidifiers can pull more current than the Nest likes.
    • Tech trick: run those through an isolation relay or a Nest Power Connector so the Nest only has to drive a tiny relay coil instead of the heavy load.
    • That often keeps an otherwise good system from constantly tripping overcurrent faults like F16.
  • Swap the base before the brain.
    If you have another compatible Nest in the house, swap just the wall base first. A cooked base can throw F16 while the display is still fine.

Is It Worth Fixing? (The Financial Verdict)

  • ✅ Fix: F16 started after recent wiring work, a thermostat swap, or visible cable damage, and your HVAC system is under about 12 years old with an estimate under roughly 250 dollars.
  • ⚠️ Debatable: F16 comes with other issues (weak airflow, noisy outdoor unit, breaker trips), the tech is talking control board plus contactor or valves, and the system is in the 12 to 15 year range.
  • ❌ Replace: You are staring at multiple big-ticket parts (control board, transformer, contactor, maybe compressor or heat exchanger), total quotes near or over 800 to 1,200 dollars, and the furnace or AC is 15-plus years old or already unreliable.

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