Boiler/furnace will not start, display shows F.00, no ignition attempt is heard

Safety first: Turn off power to the furnace at the breaker and close the gas isolation valve before removing any covers. Allow the appliance to cool fully.

Steps to check:

• Remove the front casing to access the primary heat exchanger flow pipe where the flow NTC sensor is clipped or screwed in (often labelled as flow sensor or NTC on Vaillant combi/system boilers).
• Visually inspect the flow NTC: check for broken body, damaged cable, or signs of overheating.
• Check the wiring harness from the NTC to the main PCB: ensure the plug is fully seated, no corrosion, and no pinched or cut wires.
• With power still off, disconnect the NTC and measure its resistance with a multimeter. Compare to Vaillant service data (typically around 10 kΩ at 25°C; resistance should change smoothly with temperature).
• If the NTC is open circuit (infinite resistance) or far out of spec, replace the flow NTC sensor with a Vaillant-compatible part.
• If the sensor tests good, inspect the PCB connector for bent pins or dry joints. Reseat the connector and restore power to test.

When to call a professional: If the sensor and wiring are sound, the fault may be on the main control board. PCB diagnosis and replacement should be carried out by a qualified gas-safe technician.