Boiler locks out and displays F.00, no heating or hot water

Safety first: Isolate the boiler from the mains power and close the gas isolation valve before removing the case. Only Gas Safe / certified technicians should work on gas-carrying components.

• Power reset: Turn the boiler off at the fused spur for 2–3 minutes, then power back on to see if the code clears.
• Check system pressure: Confirm the system pressure is around 1.0–1.5 bar when cold. Low pressure can cause temperature sensing issues; top up via the filling loop if needed and reset the boiler.
• Inspect flow NTC sensor: Remove the front cover (if not room-sealed or if you are qualified). Locate the flow temperature sensor (usually clipped into the flow pipe near the heat exchanger). Check for broken wires, loose plug, or signs of water ingress.
• Test the sensor: With power isolated, disconnect the NTC and measure its resistance with a multimeter. Compare to Vaillant’s temperature/resistance chart (e.g. around 10kΩ at 25°C). If open circuit or far out of range, replace the flow NTC sensor.
• Check wiring to PCB: Trace the harness from the sensor back to the main PCB. Look for damaged insulation, pinched cables, or corroded connectors. Reseat the plug on the PCB.
• If still faulty: If the sensor and wiring test OK, the main PCB sensor input may be defective. This requires diagnosis and replacement by a qualified boiler technician.