Glow-worm F6 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F6 fault code mean?
The F6 code on a Glow-Worm boiler indicates that the control board has detected a problem with the central heating flow thermistor — a small NTC (negative temperature coefficient) temperature sensor that monitors the temperature of water circulating through the heating circuit. When this sensor fails, gives an out-of-range reading, or loses its connection to the PCB, the boiler can no longer regulate heating temperatures reliably and will lock out as a precaution. The fault may also be triggered by a wiring issue between the sensor and the PCB, or — on HXi models specifically — by incorrectly connected flow and return pipes or a system pump running in reverse.
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 thermistor (NTC sensor) Common
The flow thermistor itself has failed — either gone open-circuit or drifted so far out of specification that the PCB no longer trusts it. A healthy sensor reads approximately 12kΩ at 20°C; a reading well outside this range on a multimeter confirms the sensor needs replacing. This is the single most common trigger for F6.
- Loose, corroded, or damaged wiring connections Common
The wiring harness that links the thermistor to the main PCB can develop loose pins, corroded terminals, or damaged insulation — particularly in older boilers or those in damp locations. This can produce an intermittent or permanently incorrect signal even when the thermistor itself is still serviceable.
- Circulation fault causing excessive temperature differential Sometimes
If the circulating pump is failing, partially seized, or running at too low a speed, or if there is a significant blockage in the system, the temperature difference between the flow and return becomes abnormally large. The boiler may interpret this as a sensor fault and raise F6, when the underlying problem is actually a hydraulic one.
- Reversed flow and return connections (HXi models only) Rare
On Glow-Worm HXi models, if the flow and return pipework was connected incorrectly during installation or a previous repair, the thermistor will never see the temperatures the PCB expects, causing a persistent F6. Similarly, an external system pump wired or installed to circulate water in the wrong direction can produce the same effect.
How to fix it
- Note the fault and attempt a single boiler reset DIY safe
Press and hold the reset button as described in your user manual. If the boiler fires up and runs normally, monitor it closely. If the F6 returns within a short period — or comes back every time the boiler lights — a reset alone will not resolve the underlying fault and further investigation is needed. Avoid resetting more than two or three times, as repeated lockouts can stress internal components.
- Check that your gas supply is working DIY safe
Confirm other gas appliances in your home (hob, gas fire) are working normally. If they are not, contact your gas supplier rather than your boiler engineer, as the issue lies upstream of the boiler.
- Visually inspect for obvious wiring damage (outer casing only — do not open the boiler) DIY safe
Without removing the boiler casing or touching any internal components, check whether you can see any obviously burnt, melted, or disconnected wiring near the boiler casing exterior. Report anything you notice to the engineer — do not attempt to reconnect or repair wiring yourself.
- Have a Gas Safe engineer test the thermistor resistance Gas Safe engineer
An engineer will remove the boiler casing and use a multimeter to test the flow thermistor's resistance. A reading significantly different from the expected ~12kΩ at 20°C confirms the sensor has failed and needs replacing. This work requires access to live boiler components and must be carried out by a Gas Safe registered engineer.
- Have the engineer inspect and re-seat wiring connections Gas Safe engineer
If the thermistor tests within acceptable limits, the engineer will inspect the wiring harness and connector pins for corrosion, looseness, or damage. Cleaning terminals or replacing a damaged loom section may resolve the fault without needing any new parts.
- Have the engineer assess circulation and pump performance Gas Safe engineer
If neither the thermistor nor the wiring is obviously at fault, the engineer will check system flow rate, pump operation, and look for blockages or scale build-up that could be causing an abnormal temperature differential across the heat exchanger.
- Replace the faulty component as identified Gas Safe engineer
Once the root cause is confirmed, the engineer will replace whichever component has failed — most commonly the NTC thermistor, but potentially a section of wiring loom, the circulating pump, or in rarer cases the PCB if it has developed a faulty input channel.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All diagnostic and repair work inside the boiler casing must be carried out by a Gas Safe registered engineer. You can find one at the Gas Safe Register website (gassaferegister.co.uk). Ask them to check for the F6 fault on your Glow-Worm boiler and provide a written quote before work begins.
Parts you may need
- Glow-Worm flow NTC thermistor (NTC sensor) · from £25
- Thermistor wiring harness / loom · from £35
- Circulating pump (compatible replacement) · from £85
- Main PCB (if input channel failure confirmed) · 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 £120–£320, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow-Worm F6 fault myself?
The investigation and repair work for F6 all takes place inside the boiler casing, which means it must be carried out by a Gas Safe registered engineer — it is a legal requirement in the UK. As a homeowner, you can safely try a single reset and check that your gas supply is on, but beyond that the fault needs a professional. The good news is that a thermistor replacement is one of the cheaper boiler repairs, so there is no need to put it off.
How much does it cost to fix a Glow-Worm F6 fault?
For most homeowners, the repair falls in the range of £120–£320. If the flow thermistor itself has failed, the part costs around £20–£40 and labour takes less than an hour, so the total bill is typically at the lower end of that range. If a circulating pump needs replacing, expect to pay more — usually £200–£350 including parts and labour. In the rare situation where the main PCB has developed a faulty sensor input, the PCB alone can cost £200–£350 for the part, pushing the overall bill higher, but this outcome is uncommon with an F6 code.
Why does my Glow-Worm boiler keep showing F6 and then clear itself?
An intermittent F6 that resets and then returns is a classic sign of a wiring or connection problem rather than an outright component failure. Loose or corroded pins on the thermistor connector can cause the signal to drop in and out — particularly when the boiler heats up and metal expands slightly. Although the fault clears, it will keep coming back and will eventually become permanent. It is worth getting an engineer to inspect and re-seat the wiring connections sooner rather than later, as it is usually a straightforward fix.
Is the F6 fault dangerous?
F6 is a sensor fault rather than a gas or combustion safety issue, so it does not present an immediate danger in the way that a carbon monoxide or gas leak would. However, because the boiler cannot accurately monitor water temperature, it may overheat the system or operate inefficiently before locking out. Repeated lockouts and thermal stress are not good for the boiler long-term, so you should get the fault investigated promptly rather than relying on repeated resets to keep the heating going.