Nest thermostats are smart devices that rely on low-voltage wiring, stable power, and clear communication with your heating and cooling equipment. When something in that chain goes wrong—such as a loose wire, overloaded transformer, or failed Heat Link—the thermostat throws an error code to protect your system and help you pinpoint the fault.
This guide brings together the most common Nest thermostat F‑series error codes (F2–F20) so you can quickly identify what each one means, when it’s safe to troubleshoot yourself, and when it’s time to call a professional.
Summary of Nest Thermostat Error Codes
| Code | Meaning |
|---|---|
| F2 | General wiring or power issue affecting communication between the Nest display and your system. |
| F3 | Heat Link fault or loss of communication between the thermostat and Heat Link. |
| F4 | Serious power or wiring fault preventing the Nest from controlling heating or cooling. |
| F5 | Thermostat unable to connect reliably to the Heat Link or control system. |
| F6 | Incorrect wiring configuration or incompatible system wiring detected. |
| F7 | Power-sharing or C‑wire issue causing low or unstable power to the thermostat. |
| F8 | Power or wiring fault that may be overloading the Nest or attached equipment. |
| F9 | Internal thermostat error or intermittent wiring fault requiring inspection. |
| F10 | No power detected on the Y1 (cooling) wire. |
| F11 | Communication or power problem that can prevent reliable system control. |
| F12 | Significant wiring or transformer power fault detected by the thermostat. |
| F13 | Detected short, miswire, or incompatibility on one or more control wires. |
| F14 | Overcurrent or power surge condition affecting Nest operation. |
| F15 | (E15) Serious power fault that may indicate a short or failing transformer. |
| F16 | Internal hardware or sensor fault detected by the Nest thermostat. |
| F17 | Persistent power issue or wiring configuration problem preventing normal operation. |
| F18 | Low voltage or unstable power supply detected at the thermostat. |
| F19 | No power to the Y1 cooling wire under load, often due to transformer or wiring faults. |
| F20 | General system power or wiring error that can affect heating or cooling performance. |
F2: Nest Thermostat F2 Fix (Error Code Guide)
Error F2 usually appears when the Nest thermostat detects a problem with wiring or power delivery that prevents reliable control of your HVAC equipment. It can be caused by loose connections, incorrect wire placement, or an undersized transformer that cannot keep the thermostat powered under load.
F3: Nest Thermostat F3 Fix (Heat Link Fault)
Error F3 indicates a Heat Link fault or a loss of communication between the Nest thermostat and the Heat Link. This can result from wiring issues, a tripped breaker, interference, or a failed Heat Link module.
F4: Nest Thermostat F4 Error Code Fix
Error F4 signals a serious problem with the thermostat’s ability to send or receive power and control signals from your heating or cooling system. It often points to miswired terminals, damaged cables, or a failing system transformer.
F5: Nest Thermostat F5 Error Code Guide: Fast Fix Instructions
Error F5 means the Nest thermostat cannot maintain a reliable connection to the Heat Link or control module. This can be due to distance, interference, wiring faults, or power issues affecting either the thermostat or the Heat Link.
F6: Nest Thermostat F6 Error Code Guide (Quick Fix)
Error F6 appears when the Nest detects an unexpected or unsupported wiring configuration. It may occur if wires are inserted into the wrong terminals, if the system type was set incorrectly, or if incompatible equipment is connected.
F7: Nest Thermostat F7 Fix (Error Code Guide)
Error F7 is typically linked to power-sharing issues, such as missing or poorly connected C‑wire, that leave the Nest underpowered. The thermostat may reboot, behave erratically, or lose connection when the system turns on.
F8: Nest Thermostat F8 Fix: Power/Wiring Fault Quick Guide
Error F8 points to a power or wiring fault that could overload the thermostat or attached HVAC components. It is often associated with shorts between wires, incorrect jumper settings, or a transformer that is being pushed beyond its rating.
F9: Nest Thermostat F9 Error Code Guide
Error F9 suggests an internal thermostat error or intermittent wiring fault that the Nest cannot clearly classify. Symptoms can include random shutoffs, error messages that come and go, or inconsistent heating and cooling control.
F10: Nest Thermostat F10 Fix: No Power to Y1 (Cooling) Wire
Error F10 is raised when the Nest thermostat does not see the expected power on the Y1 cooling call wire. This usually indicates a fault in the cooling circuit, such as a blown fuse, tripped safety switch, or broken connection between the thermostat and the air conditioning equipment.
F11: Nest Thermostat F11 Fix: Fast, No-Nonsense Guide
Error F11 covers a range of communication and power problems that can disrupt how the Nest talks to your heating or cooling system. It may be caused by damaged wiring, incorrect terminal assignments, or line-voltage equipment connected where only low voltage should be used.
F12: Nest Thermostat F12 Fix (Fast Wiring/Power Fault Guide)
Error F12 indicates that the Nest has detected a significant wiring or power fault that could potentially damage the thermostat or HVAC system. Common causes include shorted wires, mismatched system voltage, or a transformer that is overheating or failing.
F13: Nest Thermostat F13 Fix (No-Nonsense Error Code Guide)
Error F13 typically appears when the Nest detects a short circuit, miswire, or incompatible control signal on one of the connected wires. This can happen after DIY wiring changes, equipment upgrades, or damage to the thermostat cable.
F14: Nest Thermostat F14 Error Code Guide (Real-World Fix)
Error F14 is often related to overcurrent, a power surge, or a serious wiring problem that interrupts the Nest’s normal operation. The thermostat may shut down to protect itself, especially if the connected equipment is drawing more current than the system is designed to handle.
F15: Nest Thermostat F15 Fix (Really E15): Fast Power Fault Guide
Error F15 (often shown as E15) signals a critical power fault, frequently caused by a dead short, incorrect wiring, or a severely overloaded transformer. Ignoring this error can lead to repeated fuse failures or permanent damage to the Nest or HVAC equipment.
F16: Nest Thermostat F16 Fix: Fast Error Code Guide
Error F16 points to an internal hardware, sensor, or power-regulation fault detected by the thermostat. While some cases are related to borderline wiring or power conditions, others may require a thermostat replacement.
F17: Nest Thermostat F17 Fix Guide
Error F17 usually means the Nest is seeing persistent power irregularities or a configuration issue that prevents it from working reliably. It may recur if wiring is only marginally within spec, if the transformer is undersized, or if the system type is set incorrectly.
F18: Nest Thermostat F18 Fix: What It Really Means And How To Clear It
Error F18 is triggered when the Nest detects low or unstable voltage at its terminals, often due to long wire runs, shared circuits, or an aging transformer. The thermostat may reboot, lose Wi‑Fi, or stop calling for heating or cooling when voltage drops too far.
F19: Nest Thermostat F19 Fix (No Power to Y1 Cooling Wire)
Error F19 is a specific version of a cooling power fault, indicating that there is no reliable power to the Y1 cooling wire when the system is under load. This commonly points to a bad contactor coil, weak transformer, broken wire, or blown low‑voltage fuse near the air handler or furnace.
F20: Nest Thermostat F20 Fix (Error Code Guide)
Error F20 is a generic system power or wiring error that affects how the Nest controls heating or cooling equipment. It often appears after wiring changes, system upgrades, or intermittent faults that cause the thermostat to lose its reference signals.
Maintenance Tips to Prevent Nest Thermostat Error Codes
- Have your HVAC system and low-voltage wiring inspected regularly, especially after any upgrades or service work, to catch loose connections, degraded insulation, or undersized transformers.
- Label all thermostat wires clearly before making changes, and always follow Nest’s official wiring diagrams for your specific system type.
- Whenever possible, provide a dedicated C‑wire from the HVAC transformer to ensure the Nest has a stable power source instead of relying on power sharing.
- Keep the Nest thermostat and any Heat Link or control modules away from sources of electrical noise, moisture, or extreme temperatures that can cause intermittent faults.
- If you are unsure about system voltage, wiring compatibility, or repeated F‑codes, stop troubleshooting and contact a qualified HVAC technician to avoid damaging your equipment.