Glow-worm F19 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F19 fault code mean?
The F19 code appears on Glow-Worm boilers when the PCB detects that the central heating NTC (Negative Temperature Coefficient) thermistor is unplugged or unreadable. The thermistor is the sensor responsible for continuously feeding water temperature data back to the boiler's control board. Without a valid signal, the PCB has no way to confirm that the central heating water is staying within safe operating limits — typically 60–80°C — so it locks the boiler out as a precaution. Left unchecked, unmonitored water temperatures could cause serious damage to internal components including the heat exchanger, pump, and gas valve.
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
- Thermistor connector loose or unplugged Common
The most frequent trigger for F19 is simply that the wiring connector on the central heating NTC thermistor has worked loose or pulled free from the sensor. Vibration from the boiler running over time can gradually loosen push-fit connectors. Because the PCB sees no signal at all rather than an out-of-range reading, it immediately locks out to protect the system.
- Faulty or failed NTC thermistor Sometimes
If the thermistor is physically connected but its electrical resistance is incorrect, the PCB will still flag F19. A Gas Safe engineer can check this with a multimeter — a healthy thermistor should read approximately 12,000 ohms at 20°C. A reading that is significantly different from this indicates the thermistor has degraded or failed and needs replacing.
- Damaged wiring or corroded connector pins Sometimes
Even if both the thermistor and PCB are sound, a break or short in the cable between them, or corrosion on the connector pins, can prevent a reliable signal reaching the board. An engineer will inspect the full wiring loom and may clean or reseat the connector pins to restore a good electrical connection before considering component replacements.
- PCB fault Rare
If the thermistor and all associated wiring test correctly but F19 persists, the PCB itself may be unable to process the incoming signal. PCB failures can result from moisture ingress (often caused by a boiler leak dripping onto the board), excessive vibration, or general age-related component failure. PCB replacement is the most costly outcome and engineers will rule out every other cause first.
How to fix it
- Do not repeatedly reset the boiler DIY safe
F19 is a hard lockout caused by a missing sensor signal. Resetting the boiler will not address the underlying problem — it will simply lock out again straight away. You can press the reset button once to confirm the fault is still present, but avoid repeated resets as this can put unnecessary strain on other components.
- Check the boiler is not exhibiting any obvious external issues DIY safe
Have a look at the boiler casing for signs of water leaks or dripping, which could indicate moisture is reaching internal components. Also make sure your gas supply is working normally — check that other gas appliances in your home are functioning. Note down any observations to share with your engineer, as this will help speed up diagnosis.
- Contact a Gas Safe registered engineer to inspect the thermistor connection Gas Safe engineer
An engineer will open the boiler casing and locate the central heating NTC thermistor. In many cases the fix is straightforward — the connector may have simply worked loose and can be firmly reseated. The engineer may also remove both ends of the connector, clean the pins, and spring them out slightly to ensure a reliable electrical contact before reassembling.
- Allow the engineer to test the thermistor with a multimeter Gas Safe engineer
If reconnecting the sensor does not clear the fault after a reset, the engineer will measure the thermistor's resistance. The expected reading is around 12,000 ohms at 20°C. A reading significantly above or below this indicates the thermistor has failed and must be replaced. Thermistors are inexpensive parts and this is a straightforward repair.
- Allow the engineer to inspect the wiring loom and connectors Gas Safe engineer
Should the thermistor itself test as healthy, the engineer will trace the wiring back towards the PCB, looking for any breaks, chafed insulation, or corroded connector pins. Damaged sections of wiring can often be repaired without replacing the entire loom.
- Allow the engineer to assess the PCB if all other components check out Gas Safe engineer
If the thermistor and wiring are both confirmed to be in good order but the fault persists, the PCB will need to be evaluated. Engineers will want to be certain before condemning a PCB. If replacement is needed, note that PCBs are not typically carried on a van — your engineer will need to order the part from a merchant, which can add 2–3 days to the repair timeline.
Parts you may need
- Central heating NTC thermistor (Glow-Worm compatible) · from £15
- Thermistor wiring harness / connector · from £20
- Glow-Worm PCB (model dependent) · 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–£300, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow-Worm F19 fault myself?
The physical checks inside a boiler — reconnecting sensors, testing resistance with a multimeter, inspecting wiring — must be carried out by a Gas Safe registered engineer. There is no homeowner DIY fix for F19 beyond noting what you can see externally and calling an engineer. Attempting to open the boiler casing and work on internal components without Gas Safe registration is both illegal and dangerous.
How much does it cost to fix a Glow-Worm F19 fault code in the UK?
Most people with an F19 fault pay somewhere between £100 and £300 all in, covering the engineer's call-out, labour, and any parts. If the fix is simply reseating a loose connector, you're likely looking at the lower end — mainly the call-out and labour charge. A thermistor replacement including parts and labour typically falls in the £120–£200 range. PCB replacement is a less common outcome but significantly more expensive, often £350–£500 depending on the boiler model; if your engineer suspects the PCB may be at fault, it is worth getting a second opinion before authorising the work.
Why does my Glow-Worm boiler keep showing F19 after a reset?
Resetting the boiler clears the lockout temporarily, but if the underlying sensor signal is still absent or incorrect, the PCB will trigger F19 again almost immediately. Repeated resets without fixing the root cause will not resolve the fault and may mask worsening damage. If F19 returns after a reset, you need an engineer to diagnose whether the thermistor is disconnected, faulty, or whether there is a wiring or PCB issue.
Will an annual boiler service help prevent the F19 fault?
Yes. During a service, a Gas Safe engineer will check all sensors and their connections, clean connector pins, and spot early signs of wiring deterioration — all of which are common triggers for F19. Regular servicing also keeps your manufacturer warranty valid, which could cover part of the repair cost if your boiler is still within its warranty period. Most boiler manufacturers require annual servicing as a condition of the warranty.