Glow-worm F10 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F10 fault code mean?
The F10 code appears when your Glow Worm boiler detects a problem with either the flow heating temperature sensor (NTC thermistor) or, on certain models, the flame detection circuit. In the majority of cases — particularly on popular ranges such as the Betacom4 and Ultracom3 — the fault points to the flow NTC thermistor, a small electrical component that continuously feeds water temperature data back to the PCB. When this sensor fails, reads out of range, or loses its connection, the boiler cannot safely regulate heating output and shuts down. On some older Glow Worm models, F10 instead flags a problem with the flame-sensing electrode, meaning the boiler cannot confirm a stable flame is present. In either case, the boiler will typically lock out and refuse to fire until the fault is resolved.
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
- Faulty or failed flow NTC thermistor Common
The flow NTC thermistor is a small, fragile sensor clipped into a pocket on the flow pipe from the heat exchanger. Even minor physical knocks, age-related degradation, or brief exposure to moisture can push its resistance reading outside the range the PCB expects, triggering an immediate F10 lockout. This is by far the most common root cause on Betacom4 and Ultracom3 models.
- Loose or corroded sensor connector Common
The NTC sensor plugs into a wiring connector that can work loose over time or corrode — particularly in boilers with any history of minor internal leaks. A poor connection produces an intermittent or absent signal that the PCB interprets as a sensor failure, even if the thermistor itself is still functional.
- Damaged sensor wiring harness Sometimes
The cable running from the NTC sensor back to the PCB can suffer from chafing, heat damage, or rodent interference. A break or partial short within this harness will generate the same F10 signal as a failed sensor, so wiring continuity needs to be confirmed before parts are replaced.
- Water ingress around the sensor pocket Sometimes
A slow drip or weep from a nearby joint can allow water to track along the sensor body and into its connector, causing corrosion or a short circuit. If there is visible limescale or rust staining near the sensor location, this is a likely contributing factor.
- Flame detection electrode fault (certain models) Sometimes
On some older Glow Worm models, F10 specifically indicates the flame-sensing electrode is not producing the expected ionisation signal. This can be caused by a contaminated or worn electrode tip, a cracked electrode body, or a damaged detection lead. If your boiler attempts to ignite, fires briefly, then locks out with F10, this is worth investigating.
- PCB input fault Rare
If the PCB's own sensing circuit for the NTC input is damaged — often caused by a power surge or prolonged exposure to a short-circuit from a faulty sensor — the board itself may misread or fail to read the sensor signal entirely. This is considerably less common and usually only diagnosed after a new sensor and wiring check have ruled out the simpler causes.
How to fix it
- Check for obvious issues and attempt a single boiler reset DIY safe
Before calling an engineer, confirm your gas supply is on and other gas appliances in your home are working normally. Then attempt one reset using the button or dial on your boiler's control panel, following your model's manual. If the F10 code clears and stays clear, monitor the boiler for a short while. If F10 returns immediately or within a few minutes, do not keep resetting — repeated resets on a sensor fault can mask the problem long enough to cause secondary damage to more expensive components.
- Visually inspect the area around the flow sensor for water leaks DIY safe
Without removing any components, take a look at the pipework and sensor area inside the boiler casing (most Glow Worm models allow access to this area with the front panel open). Look for any signs of dripping water, damp residue, limescale deposits, or corrosion near the sensor and its wiring connector. Note down anything unusual and report it to your engineer — it helps narrow the diagnosis quickly.
- Have a Gas Safe engineer test the NTC thermistor resistance Gas Safe engineer
A registered engineer will disconnect the NTC sensor and use a multimeter to measure its resistance at a known ambient temperature, then compare this against Glow Worm's specification chart for that model. A reading that falls outside the expected range confirms the sensor has failed. This test also rules out a PCB input fault before any parts are ordered.
- Have the engineer inspect wiring continuity back to the PCB Gas Safe engineer
If the thermistor tests within tolerance, the engineer will check the full wiring harness from the sensor connector to the PCB terminals, looking for breaks, shorts, or high-resistance joints caused by corrosion. A damaged connector or harness section can be repaired or replaced at relatively low cost compared to other components.
- Replace the faulty NTC thermistor or flame-sensing electrode as required Gas Safe engineer
Swapping a flow NTC is a fairly quick job — the engineer will typically drain a small amount of water from the system, remove the sensor from its pocket, fit a new manufacturer-approved part, reconnect the wiring, and refill before testing. If the flame electrode is the identified cause on your model, this is similarly straightforward but must still be carried out by a Gas Safe engineer. After replacement the boiler should be run through a full heating cycle to confirm the fault has cleared.
- If the fault persists, have the PCB diagnosed and replaced if necessary Gas Safe engineer
If a new sensor and confirmed good wiring still result in an F10 code, the PCB's NTC input circuit is likely faulty. PCB replacement is a more involved and costly repair, so a competent engineer will want to be confident all simpler causes have been eliminated first. Ensure any replacement PCB is the correct part number for your specific Glow Worm model.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
The F10 fault reliably requires professional diagnosis and parts replacement — there is no safe homeowner fix beyond the reset described above. When booking, check the engineer's Gas Safe Register card or verify their registration number at gassaferegister.co.uk before any work begins. It is a legal requirement in the UK for gas boiler work to be carried out by a registered engineer.
Parts you may need
- Flow NTC thermistor sensor (Glow Worm compatible) · from £20
- Sensor wiring harness / loom · from £35
- Flame detection / ionisation electrode · from £25
- NTC sensor connector plug and terminals · from £12
- PCB (model-specific) · 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 £100–£250, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow Worm F10 fault myself?
The only safe DIY step is a single reset attempt. Beyond that, F10 requires a Gas Safe registered engineer to test the NTC sensor resistance, check wiring continuity, and replace any faulty parts. Working on sensors, wiring, or internal boiler components yourself is unsafe and may invalidate your boiler warranty.
How much does it cost to fix a Glow Worm F10 fault in the UK?
For the most common cause — a failed flow NTC thermistor — you can typically expect to pay £100–£250 including the engineer's callout, the part (usually £13–£30), and the labour. If the fault has also damaged the PCB, that repair typically adds £250–£400 on top, though most F10 faults are resolved without needing a new board.
Why does the F10 code come back after I reset the boiler?
A reset temporarily clears the lockout signal, but if the underlying sensor or wiring fault is still present the PCB will detect it again within seconds or minutes and re-lock the boiler. Repeatedly resetting without fixing the root cause can also mask the problem long enough to stress other components, potentially increasing your repair bill. If F10 returns after one reset, stop resetting and book an engineer.
Is the F10 fault the same on all Glow Worm boilers?
Not quite. On the most widely installed current models — including the Betacom4 and Ultracom3 — F10 almost always refers to the flow NTC temperature sensor. On certain older Glow Worm models, F10 can instead indicate a flame detection or ionisation electrode fault. If your boiler attempts to light, briefly fires, then locks out with F10, mention this to your engineer as it points toward the flame-sensing circuit rather than the thermistor.