Mysa Thermostat Error Codes: H1, H2, H3, H4 and H5
By Ray Bennett · Updated 24 October 2026 · 10 min read · Checked against the maker’s manual 
Mysa thermostat error codes start with H, and on the Mysa for Baseboards the meaning depends on the model. On the V1 and V2, H2 is no heater detected and H4 (V2 and Lite) a damaged relay that cannot be cleared. On the Gen3, H1 is over-temperature, H2 under-current and H5 over-current. On Mysa for In-Floor Heating, H3 is a GFCI trip and I3 a floor sensor issue.
Mysa thermostat error codes start with an H, and what each one means depends on which Mysa you have. On the Mysa for Baseboards V1 and V2, H2 means no heater detected; on the newer Gen3 it means under-current. Mysa for In-Floor Heating adds H3, a GFCI trip, and I3, a floor sensor issue.
On the V1, V2 and Lite, Mysa says the codes point to hardware issues that need attention. Everything below comes from Mysa’s own help center articles.
Before you start
What to carry: your phone, for a photo of the display (Mysa asks for one), and a torch.
Gas or electric? Electric. Mysa for Baseboards and Mysa for In-Floor Heating are line voltage thermostats: they are wired into the heater’s own 120, 208 or 240 volt circuit.
Which Mysa is it? For a V2, Lite or V1, Mysa says the model is in the app under Settings, then Thermostat Info. Mysa’s Gen3 page gives the Gen3 model as BB-Gen3-0 or MYSA BB V3-0.
Renting? Send the photo of the display and the model to whoever arranges repairs; the wiring checks below are for the person who installed it or an electrician.
What Mysa says: this is line voltage equipment. Turn off the circuit breaker before touching any wires, and use a voltage tester to make sure the power is off, which Mysa calls the only reliable proof. Do not work live. If you are not sure the power to your heaters is off, stop and call an electrician.
H4 on a V2 or Lite: Mysa says the heater keeps running and the thermostat cannot control it. Turn off the breaker and keep it off until the thermostat is replaced. H4 on a Gen3: turn the breaker off and do not keep using the thermostat.
H3 on in-floor heating: do not bypass the GFCI. Mysa says repeated trips point to a possible electrical fault that a qualified electrician should inspect.
Which Mysa, which meaning
Mysa’s Gen3 page says plainly that the older H2 and H4 explanations do not apply to the Gen3, and Mysa’s older baseboard page says the same: its legacy meanings do not apply to the Gen3. So find your model first, then read its codes.
Mysa for Baseboards V1, V2 and Lite
H2, no heater detected (V2 and V1; the Lite does not show H2). Mysa did not detect a heater between its Load and L2/N connections. The causes Mysa lists are a loose wire in a wire nut, the heater’s load wire not connected to Mysa’s Load, a heater that has malfunctioned, or a wiring error during installation. If the error clears after the wiring check, Mysa says the thermostat is working correctly; if it persists, contact support.
H4, relay damaged (V2 and Lite). The internal relay was damaged, usually by a wiring error or by exceeding the maximum load, and Mysa says this can show days, weeks or even months after installation. It cannot be cleared: the thermostat needs replacing. Mysa’s H4 article explains that the relay is stuck closed, so the heater runs continuously whatever the setting.
Mysa says H4 typically comes from over-wattage, above 3,800 W at 240 V, 3,200 W at 208 V or 1,900 W at 120 V, or from a heater rated for a different voltage than the circuit. Its other causes are loose or damaged wiring, an incompatible wiring configuration, and a surge at installation, if the breaker tripped during the very first setup. If the cause is not fixed, a replacement may fail the same way.
Any other code on these models, Mysa says, is a hardware defect you cannot fix: contact support with a photo. Mysa’s Gen3 page adds that on the V1 and V2, H5 and H6 are relay checks, stuck open or stuck closed; neither meaning applies to the Gen3.
Mysa for Baseboards Gen3
| Code | What Mysa’s Gen3 page says | Mysa’s first step |
|---|---|---|
| H1 | Over-temperature: the thermostat detected a temperature that is too high | Breaker off; heater not blocked, thermostat not on another heat source |
| H2 | Under-current: less current than expected while delivering heat | Breaker off before any wiring check |
| H4 | Current through the heater when the output should be off | Breaker off; stop using it; contact Mysa Support |
| H5 | Over-current, at about 18 A; the Gen3 is rated for 16.7 A at most | Breaker off; add up the heaters on the circuit |
The Gen3 has no H6. Mysa’s wiring troubleshooting page lists the Gen3 with the V2 and V1 under H2 as no heater detected; either way, both start with the breaker off and the heater load connected. Mysa’s Gen3 page adds: make sure the circuit is the one you think it is, and if you are not an electrician, leave the breaker off and contact support or a licensed electrician.
Mysa for In-Floor Heating
H2, open heating circuit: the thermostat detects no heater load. Mysa lists a heating mat not connected, a loose wire connection or a damaged heating element.
H3, GFCI trip or FAULT light on: the built-in GFCI detected a ground fault and tripped for safety. Mysa lists a ground fault in the heating mat, moisture in the connections, damaged wiring, or a Mysa on a circuit that already has GFCI protection. If the FAULT light stays on indefinitely right after installation, Mysa says the GFCI can no longer safely detect current: contact support.
I3, floor sensor issue: the floor sensor is not reading correctly, from a disconnected sensor wire, a damaged or broken sensor, or a sensor resistance outside the 5K to 40K ohm range.
What you can check
Photograph the code and note the model
Take a photo of the display, and note the model and what the heater was doing when the code appeared. Mysa asks for the photo and the model whenever you contact support.
Restart it the right way for your model
Mysa gives two restarts. A soft reboot, on the V2, Lite and V1 only, is holding the up and down arrows for 10 seconds. Do not do this on a Gen3, where it starts Bluetooth pairing mode. A hard reboot works on all models: turn off the circuit breaker, wait 30 seconds, then turn it back on. Not for H4: on a V2 or Lite, Mysa says to keep the breaker off until the replacement arrives, and on a Gen3, to turn the breaker off and stop using the thermostat. After H1 or H5 on a Gen3, restore power only once the area has cooled (H1) or the load is confirmed safe (H5). These restarts come from Mysa’s baseboard page; on in-floor heating, follow the code’s own steps below. A code that comes back after a restart needs the matching check below.
H1 or H5 on a Gen3: the heater and the load
For H1, turn the breaker off, make sure the heater is not blocked and the thermostat is not sitting on another heat source, and restore power only after the area has cooled. If H1 returns, leave the breaker off and contact Mysa Support.
For H5, turn the breaker off and add up every heater on that circuit. Mysa says to keep the combined load within 4,000 W at 240 V or 2,000 W at 120 V, and not to restore power until the load is confirmed safe. The breaker’s rating alone does not confirm the heaters are compatible. Mysa says heaters should have labels showing their exact wattage.
H4 on a V2 or Lite: the click test
Before asking for a replacement, Mysa’s H4 article gives a click test. It needs the power on, so turn the breaker back on only for the test, then off again as soon as you have the result. Raise the set temperature well above the room and then well below it, three or four times, and listen to the thermostat. What the result means:
- A click, but the heat stays on: a wiring fault in the home is bypassing the Mysa, such as a melted wire nut or a short. Stop using it and contact an electrician; Mysa calls this a safety hazard outside its control.
- No click at all: the relay is likely stuck or welded shut, a hardware failure for a warranty replacement.
- Clicking returns and the heat turns off: it may have been a temporary software state; keep an eye on it.
Then turn off the breaker and contact support as Mysa says.
H3 on in-floor heating: press RESET once
Press the RESET button on the thermostat. If it trips again immediately, Mysa says there may be a ground fault in the system. A GFCI breaker or GFCI outlet upstream conflicts with Mysa’s built-in GFCI and causes constant trips; Mysa says to have an electrician fit a standard breaker. Contact an electrician if the issue persists.
H2 or I3: the wiring, with the breaker off
Mysa’s steps for H2 on the V1 and V2 are to turn off the breaker, remove the faceplate and give each wire in each wire nut a gentle tug. Then make sure the heater wires connect between Mysa’s Load and L2/N, refit the faceplate and restore power. On in-floor heating, its H2 and I3 steps check the heating mat wires on the LOAD terminals and the floor sensor wires on the sensor terminals. If you installed a backup floor sensor, Mysa says you can switch to it without getting under the floor.
This is wiring on a mains circuit. On a Gen3, Mysa says that if you are not an electrician, leave the breaker off and contact support or a licensed electrician. On the other models, leave it to whoever installed the thermostat or an electrician unless you are confident, and always prove the power is off with a voltage tester first.
What each one takes
| What you see | What the work involves |
|---|---|
| A code that clears after a restart | Nothing more; note it in case it returns |
| H2 on a V1, V2 or Gen3, or on in-floor heating | A wiring check with the breaker off, by the installer or an electrician |
| H4 on a V2 or Lite | Breaker off; a replacement thermostat, and the load or wiring fixed first |
| H1 or H5 on a Gen3 | Breaker off; a clear heater, or less load on the circuit |
| H4 on a Gen3 | Breaker off; Mysa Support |
| H3 on in-floor heating | One reset; an electrician if it trips again |
| I3 | The floor sensor wiring, or the backup sensor |
Questions people ask
What does H2 mean on a Mysa thermostat?
On the Mysa for Baseboards V1 and V2, no heater detected between Load and L2/N; Mysa’s possible causes include a loose wire nut or the heater wire not on Load. On the Gen3 it means under-current, and on in-floor heating an open heating circuit.
What does H3 mean on a Mysa?
On Mysa for In-Floor Heating, the built-in GFCI detected a ground fault and tripped. Press RESET once, and have an electrician look if it trips again. Do not bypass the GFCI.
What does H4 mean on a Mysa?
On the V2 and Lite, a damaged relay: the heater may keep running. Turn off the breaker and contact support for a replacement. On the Gen3, current is flowing when the output should be off; turn the breaker off and contact Mysa Support.
Can I clear a Mysa error code by restarting it?
On a baseboard Mysa, a restart is one of Mysa’s steps: a hard reboot is the breaker off for 30 seconds. Not for H4: on a V2 or Lite it cannot be cleared and the breaker stays off until the replacement, and on a Gen3 Mysa says to turn the breaker off and not to try to clear H4 by cycling the app alone. After H1 or H5 on a Gen3, restore power only once the area has cooled or the load is confirmed safe.
What does H5 mean on a Mysa Gen3?
Over-current, at about 18 A. Turn the breaker off and keep the heaters on that circuit within 4,000 W at 240 V or 2,000 W at 120 V before restoring power.
The other electric heat guides: baseboard heater not working, baseboard heater won’t turn off, heated floor not working and Sinope thermostat error codes.
Sources: Mysa help center: error codes on Mysa for Baseboards (V1, V2, Lite), Gen3 error codes, understanding and resolving the H4 error code, baseboards installation and wiring troubleshooting, and Mysa for In-Floor Heating installation and wiring troubleshooting
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