Display shows A04, burner does not start, fan may run continuously or not at all.

Safety: Isolate electrical power before accessing the control compartment.

• 1. Power cycle: Turn the unit off for 30 seconds and back on. If A04 clears temporarily, note when it returns (during operation or at startup).
• 2. Locate flue NTC: On Ferroli water heaters, the flue temperature sensor is usually clipped to the flue hood or on the outlet of the heat exchanger. Check that it is firmly attached and not physically damaged.
• 3. Inspect wiring: Follow the two-core cable from the sensor to the PCB. Look for cuts, melted insulation, or loose connectors. Reseat the plug on the PCB.
• 4. Resistance check (for technicians): With power off and wires disconnected, measure the NTC resistance and compare with Ferroli’s temperature/resistance chart. Replace the sensor if readings are out of range.
• 5. PCB check: If a known-good sensor still gives A04, the PCB sensor input may be defective and the board will need replacement by a qualified technician.