Water heater does not start; display shows F.00 and burner never ignites

Safety first: Switch the appliance OFF at the main power switch and isolate from the electrical supply. Close the gas isolation valve. Allow the unit to cool before opening the casing. Only a qualified gas engineer should work on gas-carrying components.

• Access the sensor: Remove the front cover of the Vaillant water heater according to the installation manual. Locate the flow temperature NTC sensor on the flow pipe/heat exchanger outlet (often clipped or screwed into a pocket).
• Visual inspection: Check the NTC body and wiring for damage, burns, or signs of moisture ingress. Ensure the plug is fully seated on the sensor and on the main PCB.
• Check resistance: With power OFF, disconnect the NTC and measure its resistance with a multimeter. Compare to Vaillant’s temperature/resistance chart (e.g. around 10 kΩ at 25°C – refer to the specific model manual). If reading is open circuit or far out of range, replace the sensor.
• Inspect wiring: Trace the cable from the NTC to the PCB. Look for pinched, cut, or rubbed-through insulation. Repair or replace the harness if continuity is not present.
• Reconnect and test: Refit or replace the NTC, reconnect all plugs, restore power and gas, then reset the appliance. Run hot water and check if F.00 clears and burner ignites normally.
• If fault persists: The sensor input on the main control board may be defective. Do not bypass the sensor. Contact an authorised Vaillant service technician to test and, if necessary, replace the PCB.