Heater starts but shuts down with F04; may light briefly then extinguish.

Safety first: Turn off gas and power before removing burner covers. Flame instability can lead to incomplete combustion and carbon monoxide.

• Check for wind issues: If the heater is installed in a windy location, strong gusts can blow out the flame. Inspect for missing wind baffles or shields and consider adding wind protection per Thermeau installation guidelines.
• Clean burner assembly: Remove the burner tray and inspect for rust flakes, soot, or insect nests. Clean with a soft brush and vacuum. Ensure burner ports are clear and uniform.
• Inspect flame sensor wiring: Confirm the flame sensor lead is firmly connected to the control board and not damaged or shorted to ground.
• Check gas supply and regulator: Ensure the gas line size and regulator are correct for the Thermeau model. If other appliances cause pressure drops when they start, a gas professional should measure and adjust pressure.
• Test operation: Reassemble, restore gas and power, and call for heat. Observe flame: it should be steady and blue. Persistent F04 indicates a need for professional gas pressure and combustion analysis.