Error F.01

Safety precautions:

1) Before you do anything, turn the boiler off at the mains and isolate the electrical supply. If you are unsure how to do this, stop and call a qualified engineer. Do not work on the gas supply or burner components — any gas-side work must be done by a Gas Safe engineer.

2) Allow the boiler to cool. Work on a cold appliance to avoid burns. Wear eye protection and basic hand protection when probing inside the casing.

Initial checks a homeowner can do:

1) Note the exact error code and behaviour (steady F.01, intermittent, appears after a restart). Take a photo or write it down for the engineer.

2) Try a simple reset: switch the boiler off at the front panel and mains, wait 30 seconds, restore power and select reset per the manual. If the fault clears and does not return, monitor for recurrence.

3) With the power off and the outer case removed, visually inspect the flow NTC wiring and its connector: look for loose connectors, obvious cable damage, chafing, moisture or corrosion at plug/pins.

4) Check that the NTC connector on the PCB is fully seated. Sometimes a plug has worked loose and a reseat will cure an intermittent open circuit.

Specific diagnostic steps (requires basic electrical tools: multimeter):

1) Confirm power isolated. Remove the boiler outer cover following the manufacturer’s instructions.

2) Locate the flow NTC sensor. On Vaillant combi boilers it is typically clipped to the flow pipe/heat exchanger flow outlet and has a 2‑pin connector with a thin thermistor cable routed to the PCB.

3) Visually inspect the sensor and wiring for damage. Unplug the thermistor connector and inspect pins for corrosion or bent pins.

4) With a multimeter set to measure resistance (ohms), measure across the two NTC sensor pins. At typical room temperature (~20°C) the reading for many Vaillant NTCs will be in the order of about 10–12 kΩ (models vary — consult the boiler data sheet if available). The key checks: an open circuit (infinite/OL) means a break or failed sensor; a very low near‑zero resistance means a short. If the resistance is a sensible value, warm the sensor gently (for example by holding it in hand or applying warm water to the pipe) and watch the resistance fall (NTC resistance drops when warmed). If resistance does not change with temperature, the sensor is faulty.

5) If the sensor reading is open or shorted, the sensor or its cable needs replacing. If the sensor reading is plausible but the fault persists, check continuity from the sensor plug to the PCB connector to confirm there is no break in the harness. Measure continuity along the cable and check the plug pins at the PCB.

6) If wiring continuity fails or there are intermittent contacts, the wiring harness or connector must be repaired or replaced. If wiring and sensor are good but the PCB sees no signal, the PCB connector or the PCB input circuitry may be faulty — this requires a qualified engineer to test and replace parts as necessary.

Repair and re-test (recommended to be done by a qualified engineer):

1) Replace the faulty NTC sensor or wiring harness if tests indicate an open/short or inconsistent resistance. Use correct OEM parts or specified equivalents for your model.

2) Re-seat connectors, ensure secure routing and strain relief for the cable to prevent future chafing.

3) Refit the boiler cover, restore mains power, and reset the boiler following the manual. Run the heating and observe for normal operation and absence of the F.01 fault.

When to call a professional:

1) If the fault is anything beyond a simple loose connector or you are not fully comfortable isolating power and carrying out basic electrical checks, call a Gas Safe (or local certified) engineer.

2) Any replacement of sensors, wiring inside the sealed boiler unit, or suspected PCB faults should be performed by a qualified technician. Only a certified engineer should work on gas components or perform final commissioning and safety checks.

Notes and cautions:

1) Do not attempt to bypass sensors or run the boiler with the fault present — that can risk unsafe operation and may damage the appliance.

2) Replacing components yourself may affect warranty and can be unsafe if not done correctly. Keep a record of the error and the checks you have done to assist the engineer.

3) No repair cost estimates are provided here; contact a certified engineer or Vaillant service for quotes and parts.