Thermostat displays E03 and will not turn the heater on or off correctly.

Safety first: This is a line-voltage thermostat (typically 120/240 V). Turn OFF the circuit breaker and verify power is off with a non-contact voltage tester before touching any wires.

Steps to check:

• Verify wiring against Mysa diagram: Remove the thermostat from the wall plate. Confirm that LINE (L) and LOAD wires are connected to the correct terminals as per the Mysa installation guide. Ensure neutral (if used) is correctly landed.
• Check for loose connections: Tug each conductor gently to ensure it is firmly clamped in the terminal. Re-strip and re-terminate any damaged or oxidized copper.
• Confirm voltage at the box: With the thermostat removed and power ON (only if you are qualified), measure voltage between line and neutral (or line-to-line for 240 V). It should match the heater rating. If no voltage is present, the issue is upstream (breaker, junction box).
• Inspect for signs of overheating: With power OFF, look for discoloration or melting on the thermostat terminals or PCB near the line-sensing components.

If wiring and supply voltage are correct and E03 persists, the internal line-voltage detection circuit on the main control board is likely damaged. Replacement of the thermostat is recommended.