Heatline F00 Fault Code – Flow Temperature Sensor Open Circuit
The F00 code appears when your Heatline boiler's control board (PCB) cannot receive a valid temperature reading from the NTC thermistor fitted to the heating flow pipe. This sensor tells the boiler how hot the water is as it heads out to your radiators. When the PCB detects an open circuit — meaning no resistance signal is getting through — it locks out and displays F00 to flag the problem. Because the boiler is flying blind on flow temperature, it shuts down as a safety precaution.
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.
What does the Heatline F00 fault code mean?
The F00 code appears when your Heatline boiler's control board (PCB) cannot receive a valid temperature reading from the NTC thermistor fitted to the heating flow pipe. This sensor tells the boiler how hot the water is as it heads out to your radiators. When the PCB detects an open circuit — meaning no resistance signal is getting through — it locks out and displays F00 to flag the problem. Because the boiler is flying blind on flow temperature, it shuts down as a safety precaution.
Common causes
- Loose or disconnected sensor connector Common
The NTC thermistor plugs into both the sensor body and a terminal on the PCB. Either connection can work loose over time through vibration or during previous servicing — leaving the circuit open even though the sensor itself is fine.
- Failed NTC flow thermistor Common
The thermistor element inside the sensor can fail internally, breaking the circuit entirely. This produces an open-circuit reading at the PCB regardless of how good the wiring is, and the sensor must be replaced.
- Damaged or corroded wiring harness Sometimes
The cable running between the flow sensor and the PCB can develop breaks, chafing, or corroded terminals — particularly in older boilers or where condensation is present inside the casing. A break anywhere along that run will trigger the same open-circuit fault.
- Faulty PCB Rare
If the sensor, its connector, and the full wiring harness all test correctly, the fault may lie in the PCB's own sensor input circuit. This is a less common but more expensive outcome, diagnosed by a process of elimination.
How to fix it
- Check that the gas supply is on and attempt a boiler reset DIY safe
Before assuming a component failure, confirm your gas supply is live (other gas appliances working normally) and press the boiler's reset button. Hold it for the duration your model requires and see if the boiler fires up successfully. If F00 returns within minutes, a physical fault is confirmed and further investigation by an engineer is needed. Do not reset the boiler more than two or three times in quick succession.
- Inspect the NTC sensor connector at both ends Gas Safe engineer
A Gas Safe engineer will remove the boiler casing and trace the flow NTC sensor wiring from the sensor body to the PCB terminal. They will check that both plugs are fully seated, free from corrosion, and that no pins are bent or pushed back. Often a simple re-seat of the connector clears the fault immediately.
- Test the wiring harness for continuity and damage Gas Safe engineer
The engineer will use a multimeter to verify that the wire between the sensor and PCB carries a continuous signal with no breaks. They will also visually inspect the loom for chafing, heat damage, or moisture ingress that could explain an intermittent or permanent open circuit.
- Measure the NTC sensor's resistance and replace if faulty Gas Safe engineer
NTC thermistors have a predictable resistance at any given temperature. The engineer will measure the sensor's resistance and compare it against the expected value. A reading of infinite resistance (open circuit) confirms the sensor has failed internally. A replacement flow NTC sensor — sourced through the Vaillant Group parts network, which supplies Heatline components — is then fitted and the boiler retested.
- Replace the PCB if sensor and wiring are confirmed good Gas Safe engineer
If the sensor resistance tests correctly and the wiring harness shows no faults, the engineer will conclude that the PCB's own sensor-input circuit has failed. PCB replacement is the most expensive outcome of this fault and should only be carried out after the other components have been definitively ruled out.
- Call a Gas Safe registered engineer Gas Safe engineer
All of the diagnostic and repair work for F00 beyond a simple reset must be performed by a Gas Safe registered engineer. Do not attempt to open the boiler casing, handle wiring, or remove components yourself. You can verify an engineer's registration at the official Gas Safe Register website before booking.
Parts you may need
- NTC flow temperature thermistor (Heatline/Vaillant Group compatible) · from £18
- Wiring harness/loom (flow sensor to PCB) · from £35
- Main PCB · from £180
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–£280, depending on the underlying cause.
Frequently asked questions
Can I reset the boiler myself when F00 appears?
Yes — pressing the reset button once or twice is a perfectly reasonable first step and is safe for a homeowner to do. If the F00 code disappears and the boiler runs normally, keep an eye on it. If the fault returns quickly or the boiler won't reset at all, stop resetting and call a Gas Safe engineer, as repeated resets without resolving the underlying fault can mask a developing problem.
How much does it typically cost to fix a Heatline F00 fault?
Most homeowners with an F00 fault pay somewhere between £90 and £280 all-in for parts and labour. A loose connector that simply needs re-seating sits at the lower end, while an NTC sensor replacement usually lands in the £100–£200 range. A PCB replacement is the costlier exception — parts alone can be £150–£250 or more, pushing the total bill higher. If your boiler is older and a PCB is needed, it's worth getting a quote for a replacement boiler at the same time for comparison.
Why does Heatline use F-prefix fault codes like Vaillant?
Heatline is part of the Vaillant Group, and their boilers share the same underlying technology platform. That means the fault-code structure, many internal components, and the parts supply chain are all closely aligned with Vaillant. An engineer familiar with Vaillant boilers will generally be well-equipped to work on a Heatline appliance.
Could a lack of annual servicing cause the F00 fault?
Indirectly, yes. During a yearly service a Gas Safe engineer checks sensor connections, inspects wiring, and cleans components — catching loose connectors or early signs of corrosion before they trigger a lockout. Skipping services over several years increases the chance that a minor wiring or connector issue develops unnoticed until it causes a fault like F00.