Error H91; heater may not start or may shut down unexpectedly.

Safety: Turn off power at the breaker before opening the electrical compartment.

• Check supply voltage: Use a multimeter to measure voltage at the heater terminals. Compare to the rating plate (e.g., 120 V or 230 V).
• Inspect wiring: Ensure that the line, neutral, and ground connections are tight and correctly wired.
• Dedicated circuit: If the heater shares a circuit with large appliances, consider moving it to a dedicated circuit to avoid voltage drops.
• Check for signs of damage: Inspect the internal power supply section of the control board for burnt components or discoloration.
• Stabilize supply: If the building has unstable power, consult an electrician about surge protection or voltage stabilization.
• Board replacement: If external power is stable but H91 persists, the control board's internal power supply may be faulty and require replacement.