Glow-worm F13 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F13 fault code mean?
The F13 code appears on Glow-Worm boilers when the main PCB detects a problem with its own connections or with the signals it receives from associated components such as temperature sensors and the wiring harness. In plain terms, something in the communication chain between the circuit board and the rest of the boiler has broken down — whether that is a loose connector, a degraded sensor, or a fault on the board itself. As a safety measure the boiler locks out and will not restart until the underlying issue is identified and corrected. The precise trigger can vary slightly across model ranges (CXI, HXI, SXI, Flexicom, Betacom, Energy), so if the standard checks below do not clear the fault, a Gas Safe engineer will need to carry out hands-on diagnostics.
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
- Loose or corroded wiring harness connector Common
Years of vibration and heat cycling inside the boiler can work connectors loose from the PCB or from sensors. Even a slightly unseated multi-pin connector can break the signal path and trigger F13. Corrosion on the pins — especially in boilers installed in damp utility rooms or garages — produces the same effect without any visible damage to the wire itself.
- Faulty or ageing PCB Common
The main PCB is the brain of the boiler and, like any electronic board, it ages. Dry solder joints, heat-damaged tracks, or moisture ingress can all cause intermittent or permanent communication failures. F13 faults are notably more frequent on CXI, HXI, and SXI models that are ten years old or more, where component degradation is well advanced.
- Failed temperature sensor Sometimes
The flow or return temperature sensor sends continuous readings to the PCB. If a sensor develops an open circuit, short circuit, or simply drifts out of its calibrated range, the PCB cannot trust the data it receives and shuts the boiler down. This can look identical to a PCB fault until an engineer tests each sensor with a multimeter.
- Chip card or configuration fault Sometimes
Some Glow-Worm models use a removable chip card (or configuration EEPROM) that stores boiler-specific settings. If this card is missing, incorrectly seated, or has become corrupted, the PCB will not initialise correctly and may display F13 on start-up.
- Moisture or heat damage to electronics Rare
Condensation from a nearby leak, a poorly sealed flue, or simply a boiler located in a very humid environment can cause short circuits on the PCB or at connector pins. Heat stress from a previous overheating episode can also degrade electronic components over time without leaving obvious physical signs.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame symbol or labelled 'Reset') for around three seconds, then release and wait up to two minutes for the boiler to attempt a restart. If F13 clears and the boiler runs normally, monitor it over the next 24 hours. If the fault returns, do not keep resetting — repeated resets will not fix an underlying electronic fault and may mask useful diagnostic information.
- Check the boiler's system pressure DIY safe
Glance at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, top it up via the filling loop (usually a silver braided hose under the boiler) until the needle sits around 1.2 bar. Low pressure is more commonly associated with other fault codes, but ruling it out takes seconds and costs nothing.
- Verify the gas supply is live DIY safe
Check that other gas appliances in your home (hob, gas fire) are working normally. If nothing gas-powered is working, contact your gas supplier or call the National Gas Emergency Service on 0800 111 999. If gas is available elsewhere, the supply to the boiler is not the issue.
- Arrange a professional inspection of wiring and connectors Gas Safe engineer
A Gas Safe engineer will open the boiler casing and systematically check every connector on the wiring harness — both at the PCB end and at each component (sensors, gas valve, fan). Loose or corroded pins are cleaned, re-seated, or replaced. This is the most common fix for F13 and is often completed within a single visit.
- Professional sensor testing and replacement if required Gas Safe engineer
Using a multimeter, the engineer will measure the resistance of the flow and return temperature sensors across a range of temperatures and compare the readings against manufacturer specifications. A sensor reading outside tolerance is replaced with a genuine Glow-Worm part. This is a straightforward swap once the faulty sensor is identified.
- PCB inspection, repair, or replacement Gas Safe engineer
If the wiring and sensors test as healthy, attention turns to the PCB itself. The engineer will look for dry solder joints, burnt tracks, or moisture damage. In some cases a repair is possible; in others the board must be replaced. Given the age at which F13 typically appears on older CXI/HXI/SXI models, it is worth asking the engineer to give an honest assessment of whether repair costs are justified against the remaining useful life of the boiler.
- Call a Gas Safe registered engineer if the fault persists or you have not already done so Gas Safe engineer
All of the diagnostic and repair work for F13 requires a Gas Safe registered engineer. You can find one at GasSafeRegister.co.uk. Ask them to confirm their registration number before work begins, and ensure you receive a signed job sheet on completion.
Parts you may need
- Glow-Worm main PCB (model-specific) · from £180
- Flow temperature sensor (NTC thermistor) · from £25
- Return temperature sensor (NTC thermistor) · from £25
- Wiring harness / loom · from £55
- Chip card / configuration card · from £40
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 £150–£400, depending on the underlying cause.
Frequently asked questions
Is F13 on a Glow-Worm boiler dangerous?
F13 is not a gas-leak or carbon monoxide warning — it is an electronic communication fault. However, you should leave the boiler switched off at the programmer and call a Gas Safe engineer, because the root cause (damaged wiring, a failing PCB) should be diagnosed properly rather than bypassed with repeated resets.
My Glow-Worm CXI is over 10 years old and showing F13 — is it worth repairing?
F13 on older CXI, HXI, and SXI models is often a PCB replacement job, which can cost £300–£400 for parts and labour. At that price point it is worth asking your engineer for an honest assessment: if the boiler is also due a service, has other worn parts, and its efficiency is poor, a new boiler may offer better long-term value. Modern A-rated condensing boilers are significantly more efficient and typically come with 5–10 year manufacturer warranties.
Can I fix F13 myself to save money?
The only safe DIY actions are a single boiler reset, checking system pressure, and confirming your gas supply is on. Everything else — inspecting connectors, testing sensors, or touching the PCB — must be carried out by a Gas Safe registered engineer. Working on the internal components of a gas boiler without Gas Safe registration is illegal and potentially dangerous.
How long does an F13 repair typically take?
If the fault is a loose connector or a failed sensor, a Gas Safe engineer can usually diagnose and fix it in a single visit lasting one to two hours. PCB replacement may require the engineer to order a part if they do not carry it in their van, adding a day or two before the boiler is back in service. Either way, most F13 repairs are resolved within 48 hours of an engineer's first visit.