Water heater locks out with F.01; unit attempts to start but shuts down quickly

Safety: Isolate electrical power and close the gas valve before removing covers. Allow the appliance to cool.

• Locate the flow NTC: On Vaillant water heaters, the flow temperature sensor is usually on the outlet pipe or directly on the heat exchanger.
• Check for moisture and corrosion: Remove the NTC plug and inspect for water, corrosion, or carbon tracking between pins. Dry thoroughly and clean with electrical contact cleaner if minor corrosion is present.
• Measure resistance: With the sensor disconnected, measure resistance at room temperature. A reading near 0 Ω indicates a shorted sensor. Replace the NTC if shorted or if readings do not match Vaillant’s specification.
• Inspect cable: Look for crushed or melted insulation where the cable passes near hot components or sharp edges. Repair damaged sections or replace the harness.
• Reconnect and reset: Reassemble, restore power and gas, and reset the fault. Run hot water and monitor operation.
• Persistent F.01: If a known-good sensor and wiring are present, the main PCB may be faulty. This should be diagnosed and replaced only by a qualified Vaillant technician.