Error F.00

Safety first: before doing anything isolate the boiler electrically at the fused spur or isolator switch and allow the appliance to cool. Do not work on gas or mains electrical components unless you are qualified. If the boiler is locked out, you can try a simple reset first (follow the manufacturer's reset procedure in the manual), but do not repeatedly attempt to run it if the fault persists.

Initial homeowner checks you can safely do:

1) Record the exact error code and any other lights or messages on the display. Take a photo for a service engineer if needed.

2) Try a power cycle: switch the boiler off at the isolator, wait 1–2 minutes, then switch it back on and attempt a reset. Note if the fault returns immediately or after a short time.

3) Visually inspect the externals: check for obvious water leaks, damaged cables visible outside the boiler, or corrosion around pipework. Check the system pressure gauge; very low pressure can cause other errors but is not usually F.00.

4) Locate (from the manual or parts diagram) the flow NTC position — typically clipped to the flow pipe on the main heat exchanger or the flow manifold. Without opening the boiler you can normally see its connector into the wiring loom.

If you are comfortable with basic electrical checks and the boiler is isolated from mains power, you can perform a few diagnostic tests with a multimeter (only if competent):

5) Check the connector: remove power, unplug and re-seat the NTC connector if accessible. Inspect for bent pins, corrosion, or a loose plug.

6) Continuity/visual check of wiring: visually follow the cable from the sensor to the PCB where possible. Look for chafing, cuts, or rodents bites. If the cable appears damaged, that is likely the cause.

7) Measure the NTC resistance: with the sensor disconnected from the board and at room temperature (around 18–25°C), measure resistance across the two NTC wires. Typical Vaillant flow/return NTC values are around 10–12 kΩ at normal room temperature (approx. 20°C) — exact value can vary by model. A very high/infinite reading indicates an open circuit (broken sensor or cable). A near-zero reading indicates a short. Either result confirms a faulty sensor or wiring and will require replacement.

If you find a loose connector or minor wiring fault that you can safely correct (re-seat connector, secure cable outside the boiler), reconnect, restore power and reset the boiler. If the F.00 fault clears and the boiler runs normally for an extended period, monitor for recurrence.

When to stop and call a professional:

- If you have to open the boiler casing, access gas components, or touch PCB connections. These tasks must be done by a Gas Safe registered engineer.

- If the NTC resistance is open or shorted, or you find damaged wiring: replacement of the sensor and/or harness is required and should be carried out by a qualified engineer to ensure correct parts, fitment and safety.

- If reseating connectors and a reset do not clear the fault, or the fault returns intermittently: this can indicate a wiring harness fault or PCB/electronics issue that needs professional diagnosis.

- If you are not confident using a multimeter, or if the boiler is under warranty, contact a Gas Safe engineer or Vaillant-authorised service agent.

Notes for the engineer visit: advise them of the exact code (F.00), what checks you have already done (reset, visual inspection, any resistance readings), and whether the fault is constant or intermittent. The likely fixes are replacement of the flow NTC sensor, replacement of the sensor harness, or in rarer cases repair/replacement of PCB/electronics. Do not attempt DIY replacement of sensors or parts that require boiler case removal unless you are qualified; safety and correct commissioning are important.