getting an 'err 8' code on my mysa, what does that mean and how do i clear it?

Diagnosis

The "Err 8" code on a Mysa Smart Thermostat is a critical safety trigger indicating a Voltage Out-of-Range error. This occurs when the internal monitoring circuitry detects that the incoming line voltage is either significantly lower or higher than the threshold the device is rated for. Because Mysa units are designed for high-voltage electric baseboard heating (typically 120V or 240V), any fluctuation—such as a partial power failure, a loose neutral wire, or a brownout—will trigger this lockout to prevent the internal relays from welding shut or the PCB from overheating.

This error is most common during the initial installation of a V2 unit if the voltage configuration in the app doesn't match the actual line voltage, or after a power surge. You can confirm this is a voltage issue if the screen is flickering or if the unit shuts down immediately after you attempt to clear the code and engage the heater. If you are using a multimeter, you will likely find the voltage at the Line (L) and Neutral (N) terminals is fluctuating or sitting outside the +/- 10% tolerance of your home's standard electrical supply.

How to Fix It

To resolve Err 8, you must perform a hard power cycle and verify the physical integrity of the high-voltage connections. Follow these steps precisely:

1. Locate your electrical panel and switch the circuit breaker powering the baseboard heaters to the OFF position. Wait at least 60 seconds to allow the capacitors on the Mysa board to fully discharge.
2. Carefully pull the Mysa faceplate away from the wall mounting bracket. Inspect the wiring terminals. Ensure the Line (L) and Neutral (N) wires are clamped tightly; a loose neutral wire is a primary cause of Err 8.
3. Check the wire gauge. Ensure you are using 12 AWG or 14 AWG copper wire. If the wire is frayed or the copper is oxidized, strip back 1/2 inch of insulation to expose fresh copper before re-inserting.
4. Reattach the faceplate and switch the circuit breaker back to ON.
5. Once the device boots, open the Mysa App on your smartphone. Navigate to Settings > Device Settings > Voltage Configuration.
6. Verify that the selected voltage matches your home's electrical system (e.g., 240V for most baseboards, or 120V for specific smaller units). If it is set to 120V but you have a 240V line, the Err 8 code will persist or recur.
7. Set the desired temperature and wait 30 seconds to see if the relay clicks and the error remains clear.

If That Didn't Work

• Perform a Factory Reset: If the error is a software glitch resulting from a power spike, a factory reset is required. Press and hold the reset button (or the specific button sequence for your model) for 10 seconds until the screen flashes. Re-pair the device to your Wi-Fi and re-configure the voltage settings from scratch.
• Test with a Multimeter: Set your multimeter to AC Voltage (range 600V). Carefully measure the voltage across the L and N terminals while the unit is powered. If you see a reading below 108V on a 120V circuit or below 216V on a 240V circuit, the issue is with your home's electrical supply or a failing breaker, not the thermostat.
• Inspect for "Floating Neutral": If you have multiple Mysa units on one circuit and only one is showing Err 8, check for a "floating neutral." This happens when the neutral wire is shared across multiple boxes and has a poor connection at the junction. Move the neutral wire to a dedicated terminal if possible.
• Hardware Replacement: If the voltage is confirmed stable at the terminals but Err 8 persists after a factory reset, the internal voltage sensing resistor on the PCB has likely been damaged by a surge. Contact Mysa support for a warranty replacement of the unit.

Heads Up

• Never attempt to bypass the Err 8 code by ignoring it; this is a safety feature to prevent electrical fires in your walls.
• Always ensure your baseboard heaters are rated for the voltage you have configured in the app; mixing 120V heaters on a 240V circuit will destroy the heating elements and the thermostat.
• Avoid using "daisy-chain" wiring for the power supply to the thermostat; a direct line from the junction box to the unit is required to prevent voltage drops.
• If you live in an area prone to power surges, install a whole-house surge protector to prevent the sensitive electronics in the Mysa V2 from triggering voltage errors.