Error F.01

Safety first:

- Before doing anything, turn off the boiler and isolate electricity at the fuse spur or consumer unit. If you are in the UK, only a Gas Safe registered engineer should work on gas or internal boiler components. Opening the boiler may void warranty; check your manual.

- Do not touch live circuits or probe connectors with the power on. If you are not comfortable with basic electrical checks, call a qualified engineer.

Initial homeowner checks (safe, non-invasive):

1. Note when the fault appears (on startup, when heating demands, intermittent). Write down any other fault codes or symptoms.

2. Try a simple reset: switch the boiler off, wait 1–2 minutes, switch back on and observe. If the fault reappears immediately or repeatedly, further checks are needed.

3. Visually inspect the external area around the boiler for recent work, knocks, or signs of water leak that could have damaged wiring. Look for obvious disconnected plugs on the outside of the boiler casing (do not open the casing yet).

4. Check the fault memory if your model supports it (see manual) to confirm this is consistently the same code.

If you are competent and choose to proceed (ONLY with power isolated):

1. Isolate mains power and, if applicable, gas supply. Lock off the electrical supply if possible.

2. Remove the boiler cover following the manufacturer’s instructions. Keep screws and panels safe.

3. Locate the flow and return NTC sensor connectors and their wiring harness; these are usually clipped to the heat exchanger pipes and plug into the PCB or a wiring loom.

4. Check connectors are fully seated and free of corrosion or water. Re-seat any loose plugs (with power off) and ensure clips/latches engage.

5. With the sensor unplugged (power isolated), measure the sensor resistance with a multimeter on the resistance (ohms) range. Typical Vaillant NTC values are roughly 10–12 kΩ at ~20°C (consult your boiler’s tech data for exact values). An open circuit (OL) or very high resistance indicates a failed sensor or broken conductor; a very low resistance suggests a short.

6. If the sensor reads as expected but the boiler still reports F.01, check continuity of the wiring harness back to the PCB (resistance/continuity test) to find breaks. Also inspect for signs of chafing, rodent damage, or moisture in the connector area.

7. If the sensor is defective, replace the NTC sensor only with the correct Vaillant part. If wiring is damaged, replace or repair the harness using proper connectors and insulation.

8. If wiring and sensor test OK but the fault persists, the PCB connector or PCB input stage may be faulty — this requires a qualified engineer.

Final notes and when to call a professional:

- If any step involves working with the gas supply, internal gas components, live electrical circuits, or the boiler’s PCB, stop and call a Gas Safe / certified engineer.

- Repeated lockouts after resets, intermittent faults, failed sensor replacement, or any PCB connector issues should be handled by a professional. They can test sensor characteristics accurately, replace parts safely, and update fault memory and system settings.

- Even if you can identify a disconnected plug or obviously failed sensor, fitting replacement parts inside the boiler and certifying gas safety must be done by a qualified engineer to ensure safe operation and to protect your warranty.