Nest Thermostat Error E87 — Fix Guide

What Does This Error Mean?

Nest error E87 means there is a problem with your HVAC wiring configuration. The Nest detected a wiring error that could damage your heating or cooling equipment. This usually means wires are crossed, missing, or connected to the wrong terminals.

Affected Models

Nest Learning Thermostat (all generations)
Nest Thermostat E
Nest Thermostat (2020)

Common Causes

A wire is connected to the wrong terminal on the Nest base
A required wire (such as the C-wire for power) is missing from the setup
Two wires that should not be connected are touching or crossed
The HVAC system's wiring does not match the system type selected in Nest settings
A wire became loose at the Nest base or at the HVAC equipment

How to Fix It

Check your wiring at the Nest base. Go to Settings > Equipment on your Nest to see what wires it has detected. Compare this against a photo you took of your old thermostat's wiring before installing the Nest.

If you did not take a photo of the original wiring, look for a wiring diagram on your furnace or air handler — there is usually a label inside the equipment panel.
Make sure each wire is fully inserted into its terminal. Pull lightly on each wire at the Nest base. If any come out easily, they were not fully seated. Push them back in firmly until you feel a click.

Wires that are not fully clicked in can make intermittent contact, causing wiring errors like E87.
Check the Nest Compatibility Checker. Visit nest.com/support and use the online compatibility checker with your HVAC equipment model numbers. It verifies whether your system works with Nest.

Some older or unusual HVAC systems require special wiring configurations not immediately obvious during installation.
Use the Nest app to run a new equipment setup. Go to Settings > Reset > Schedule (this keeps your settings). Then go through the wire setup again. This refreshes the Nest's wiring detection.

Re-running the equipment setup sometimes resolves false E87 errors triggered by a one-time detection glitch.
If the error persists and you cannot identify a wiring problem, call a licensed HVAC technician to verify the wiring and the system compatibility.

A technician familiar with Nest installations can quickly verify your wiring is correct and test whether the HVAC system is responding properly.

When to Call a Professional

HVAC wiring errors should be addressed carefully. If you are not comfortable with wiring, call a licensed HVAC technician. Incorrect wiring can damage your furnace, air handler, or heat pump — repairs can be expensive. Always turn off your HVAC system at the breaker before inspecting wiring.

Frequently Asked Questions

What is a C-wire and do I need one?

The C-wire (common wire) is a neutral wire that provides continuous power to smart thermostats. Older thermostats did not need it — they borrowed power from the heating/cooling signal wires. Nest thermostats can work without a C-wire using its battery, but some HVAC systems and some Nest models require one for reliable operation. Missing a C-wire is a very common cause of Nest wiring errors.

Can incorrect Nest wiring damage my furnace?

Yes. Wiring the Nest incorrectly — especially crossing the Y (cooling) and W (heating) wires — can send wrong signals to your furnace or air conditioner. This can cause the equipment to run when it should not, or fail to run when needed. Always double-check wiring against your original thermostat setup before powering on.

My old thermostat worked fine. Why does Nest show E87?

Nest is much smarter than a basic thermostat and does more extensive self-checking. It detects wiring situations that a basic thermostat would silently ignore. Something that worked fine with a dumb thermostat may need to be correctly wired to work with Nest's safety checks.