Vaillant F.00 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.00 fault code mean?
The F.00 fault code (sometimes shown as F.0 on older displays) tells you that the boiler's heating-flow NTC thermistor has an open circuit — in plain terms, the sensor that monitors the temperature of water leaving the heat exchanger has lost its connection to the control board. Because the boiler can no longer safely track flow temperature, it locks out as a precaution and will not fire until the issue is resolved. The fault sits firmly in electrical/sensor territory: the physical sensor itself, the wiring between it and the PCB, or in rarer cases the PCB itself is responsible.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Failed or open-circuit flow NTC thermistor Common
The NTC thermistor itself has broken down internally, creating an open circuit. This is the single most common root cause of F.00. The sensor is a small, inexpensive component mounted on the flow pipe at the heat exchanger, but its failure cuts all temperature feedback to the PCB, triggering immediate lockout.
- Loose, unplugged, or damaged wiring Common
The connector at the NTC plug or at the PCB can work loose over time — especially after a boiler service where components are disturbed. A damaged section of the cable harness (chafed insulation, a broken core) produces exactly the same open-circuit signal as a dead sensor. Engineers often find this is the cause when the thermistor itself tests fine.
- Faulty PCB Rare
In a minority of cases the PCB's input circuit for the flow NTC develops a fault, so even a perfectly good sensor and intact wiring return an open-circuit reading. PCB failure tends to produce other fault codes alongside F.00, which can help distinguish it from a straightforward sensor or wiring issue.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (the flame symbol with a line through it) for roughly three seconds. If F.00 clears and the boiler runs normally, monitor it over the next 24 hours. A one-off occurrence can sometimes follow a brief electrical disturbance. If the code returns, a reset alone will not cure it — move on.
- Check that your gas supply is working DIY safe
Confirm other gas appliances in the property (hob, fire) are working normally. If there is no gas, contact your supplier before doing anything else. An F.00 is not a gas-supply fault, but ruling this out takes seconds.
- Have a Gas Safe engineer inspect the flow NTC thermistor and its wiring Gas Safe engineer
An engineer will locate the heating-flow NTC on the flow pipe at the heat exchanger, disconnect it, and measure its resistance with a multimeter. A healthy NTC reads roughly 10 kΩ at 25 °C and changes predictably with temperature; an open-circuit reading (infinite resistance) confirms sensor failure. They will also check the connector and the cable run back to the PCB for continuity.
- Replace the flow NTC thermistor if faulty Gas Safe engineer
If the sensor has failed, the engineer will fit a replacement Vaillant-compatible NTC thermistor. This is typically a straightforward job: the old sensor clips or screws off the pipe, the wiring connector is swapped over, and the boiler is powered back up to verify the fault has cleared. Parts are inexpensive (around £10–£30), so the cost is predominantly labour.
- Repair or replace the wiring harness if the sensor is intact Gas Safe engineer
Should the NTC test within specification, the engineer will trace the cable harness for damage, re-seat any loose connectors at the PCB, and use a continuity tester to identify any broken cores. Damaged sections of harness may be repaired or the full loom replaced depending on where the fault lies.
- Investigate and replace the PCB if wiring and sensor are both serviceable Gas Safe engineer
If both the NTC and its wiring check out, attention shifts to the PCB. The engineer may test the board's NTC input terminals directly. PCB replacement is the most expensive outcome but is relatively uncommon for a standalone F.00 with no other fault codes present.
- Ask the engineer to clear the fault and carry out a short test run Gas Safe engineer
Once the repair is complete, the engineer should reset the boiler, run it through a heating and hot-water cycle, and confirm that flow temperature readings on the display are stable and plausible. They should also check the fault-code history to ensure F.00 is no longer logging.
Parts you may need
- Vaillant flow NTC thermistor (heating circuit) · from £18
- NTC connector / wiring plug · from £12
- Boiler wiring harness (ecoTEC) · from £55
- Vaillant PCB (model-specific) · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £90–£300, depending on the underlying cause.
Frequently asked questions
Can I fix Vaillant F.00 myself?
Not safely. The repair requires opening the boiler casing to access electrical components — specifically the NTC sensor, wiring harness, and potentially the PCB. UK law does not restrict homeowners from working on electrical components the way it does for gas work, but Vaillant strongly advise against opening the casing yourself, and doing so will void your warranty. Call a Gas Safe registered engineer; this is usually a straightforward, single-visit repair.
How much does it cost to fix F.00 on a Vaillant boiler?
Most homeowners pay between £90 and £300 all-in for an F.00 repair. The NTC sensor itself costs as little as £10–£30 for the part, but you are mainly paying for the engineer's call-out and labour (typically £30–£60 per hour). Most jobs are diagnosed and finished in one to two hours. In the rare situation where the PCB is at fault, the cost can rise to £400–£600 or more, but that outcome is uncommon for a fault showing only F.00 with no other codes.
Is F.00 the same as F.0 on my Vaillant boiler?
Yes — both refer to the same heating-flow NTC thermistor open-circuit fault. Older Vaillant displays show a single digit (F.0) while newer models display the full two-digit code (F.00). The diagnosis and repair process is identical regardless of which format appears on your boiler.
Will my Vaillant warranty cover an F.00 fault?
It can, provided you have kept up with annual boiler services by a qualified engineer — this is a standard condition of Vaillant's extended warranty. If your service history is up to date and the boiler is within the warranty period, contact Vaillant directly on 0330 100 3143 before arranging an independent engineer, as a warranty repair is free of charge. If the warranty has lapsed or annual servicing has been missed, you will need to arrange and pay for a Gas Safe engineer yourself.