Nest Thermostat F15 Fix (Really E15): Fast Power Fault Guide

by

What This Error Means

F15 on a Nest thermostat is not an official Nest code. In almost every real case, the screen is showing E15, which means a serious power or wiring fault at the thermostat. In plain terms: your Nest is seeing the wrong voltage or a short on the control wires and is shutting itself down to avoid frying the electronics (or it is already damaged).

Official Fix

  • Kill power first. Go to your breaker panel and switch off the furnace or air‑handler breaker and any outdoor unit breaker. Do not just turn the Nest off in the menu; you want real power off.
  • Pull the Nest display off the base. Grab the outer ring and pull straight toward you. Set the display aside somewhere safe.
  • Check if your system is high‑voltage.
    • Thin thermostat wires (like doorbell wire, usually multiple colors) = low voltage (OK for Nest).
    • Thick, stiff wires in a cable with wire nuts or going straight into an electrical box = high voltage (NOT OK for Nest).
    If it is high‑voltage, stop. The Nest is not compatible here. You need an electrician or HVAC tech to sort it out and a proper compatible thermostat.
  • Inspect the wiring at the Nest base.
    • Each terminal should have exactly one wire, fully inserted, clamp tight.
    • No stray bare copper should be able to touch another terminal or the metal behind the base.
    • Check that R (or Rc/Rh), W/W1, Y/Y1, G, and C match the labels on your furnace or boiler control board, or the photo of your old thermostat before you swapped it.
    • Remove any jumper wires you added. Nest does not use external jumpers between R, Rc, Rh, etc.
  • Re‑seat and clean up every wire. Open each spring clamp, pull the wire out, strip to about 6 mm (1/4 inch) of clean copper if needed, and push it back in straight. Make sure the insulation is right up to the terminal.
  • Reattach the display and restore power. Snap the Nest back onto the base, then turn the breakers back on.
  • Watch the screen. If the code (E15/F15) does not come back and heating and cooling both run, you got lucky. The thermostat survived, and the wiring fix did it.
  • If the error returns immediately or the Nest keeps rebooting or going dead: the official call from Google is: stop using it. A surge, short, or wrong wiring probably damaged the thermostat. At that point, the proper move is to contact Nest or Google support or your installer and plan on a replacement thermostat after the wiring problem is corrected.

The Technician’s Trick

  • Check the furnace or air‑handler fuse, not just the Nest. With power OFF, remove the blower door panel, find the main control board, and look for a small 3–5 amp automotive‑style blade fuse on the low‑voltage side. If it is blown, replace it. A slip of the screwdriver or touching R to C while wiring the Nest can pop this fuse and trigger power faults.
  • Meter the 24 volts at the source. If you have a multimeter and know how to use it:
    • Power ON, set meter to AC volts.
    • Measure between R and C on the furnace or boiler control board.
    • You want roughly 24 VAC. If there is no 24 VAC, you have a system power problem, not a thermostat problem.
    • If you have 24 VAC at the board but the Nest still shows E15/F15 or stays dead, the Nest electronics are usually cooked.
  • Throw on a cheap dumb thermostat as a test. Pull the Nest base off and temporarily land R and W (and Y/G if you want to test cooling or fan) on a basic non‑programmable thermostat.
    • If the system heats and cools normally with the cheap stat, your Nest is the bad part. Replace it and fix the wiring before you put the new one on.
    • If the system still will not run, you are chasing an HVAC issue (board, transformer, safety switch), not a Nest issue.
  • Do not try to save a clearly fried Nest. If you saw a flash or spark while wiring, or you know the system was high‑voltage, assume the Nest took the hit. Replace any blown low‑voltage fuse, clean up or redo the wiring, add a proper C‑wire or Nest Power Connector if needed, and then install a new thermostat. Trying to nurse a damaged Nest just wastes time.

Is It Worth Fixing? (The Financial Verdict)

  • ✅ Fix: Wiring mistake is obvious, maybe a cheap fuse blew, the HVAC system is under about 12–15 years old, and you only need to tidy wiring and possibly replace the thermostat once.
  • ⚠️ Debatable: The Nest is dead and the furnace or air‑handler control board or transformer also looks cooked, so parts and labor are heading into the several‑hundred‑dollar range on a mid‑life (15–18 year old) system.
  • ❌ Replace: The system is 18–20+ years old, has multiple electrical issues, and now needs a control board plus a new thermostat; at that point, it is usually smarter to put that money toward a new, efficient system with a fresh warranty and modern controls.

Parts You Might Need

As an Amazon Associate we earn from qualifying purchases.