Mysa No Line Voltage / Internal Power Supply Failure

Thermostat will not power on at all (no display, no lights) after installation.

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Safety Warning

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

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Possible Causes

No power at circuit breaker, Incorrect wiring of LINE and LOAD, Failed internal power supply on thermostat PCB

How to Fix / Troubleshooting

Safety first: This is a line-voltage device. Turn OFF the breaker before removing the thermostat. Only test live voltage if you are qualified.

Steps to diagnose:

  • Check breaker and wall switch: Ensure the correct breaker is ON and any wall switches controlling the heater circuit are ON.
  • Verify line voltage at box: With the thermostat removed and using a multimeter (if qualified), confirm that rated voltage (e.g., 120/240 V) is present at the LINE conductors.
  • Confirm wiring: Make sure LINE wires are connected to the thermostat’s LINE terminals, not to LOAD, and that connections are tight.
  • Test with a known-good device: If possible, temporarily connect a test lamp or another thermostat to confirm the circuit is live.

If proper voltage is present at the LINE terminals and wiring is correct but the thermostat remains dead, the internal power supply has likely failed. Replace the thermostat.

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Repair Difficulty

Hard 4/5

Required Part

Internal Power Supply / Main Control Board
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