Glow-worm F17 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F17 fault code mean?
The F17 code appears on Glow-Worm boilers when the control electronics detect that the incoming mains electricity has dropped below approximately 170V — the minimum threshold the appliance needs to run safely. Rather than a fault inside the boiler itself, this is almost always a problem with the electrical supply reaching the unit. Because the boiler's pump, fan, ignition circuitry, and safety systems all depend on a stable mains supply, the appliance locks out to prevent damage from running on under-voltage. The fix may sit anywhere between the street supply and the boiler's own internal fuse or PCB, so a logical, staged approach is needed.
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
- Wider mains supply problem Common
A low or unstable voltage from the street — caused by a local grid issue, heavy demand on the supply, or a fault at the meter — is one of the most frequent triggers for F17. If lights flicker or other appliances behave oddly elsewhere in the property, this is the likely culprit.
- Tripped circuit breaker or switched-off fused spur Common
If the boiler's dedicated fused spur has been switched off, or the circuit breaker in the consumer unit has tripped, the boiler will receive no usable voltage. This is a straightforward check and easy to resolve before calling anyone out.
- Loose or degraded electrical connections Sometimes
A poor connection — at the fused spur, junction, or within the boiler's wiring loom — can cause the effective voltage at the appliance to fall below the safe threshold, particularly under load. These intermittent faults can be tricky to track down without a meter.
- Blown internal fuse Sometimes
A power surge on the mains can cause the boiler's internal fuse to blow, protecting downstream components. With the fuse gone, the board sees insufficient voltage and throws F17. A Gas Safe engineer can check and replace this fuse safely.
- Faulty PCB Rare
If the mains supply, fused spur, wiring, and internal fuse all test correctly, the PCB itself may be misreading the supply voltage or have suffered surge damage. PCB failure is less common but should be considered once all other causes are ruled out.
How to fix it
- Check other electrical appliances around the home DIY safe
Flicking lights, appliances resetting, or sockets losing power all point to a wider supply issue rather than a boiler-specific fault. If you notice these signs, contact your electricity supplier or network operator — there may be a known outage or voltage problem on your street.
- Inspect the consumer unit and the boiler's fused spur DIY safe
Head to your consumer unit (fuse box) and look for any tripped breakers — a tripped switch sits in the middle position rather than fully up or down. Reset it if safe to do so. Also check the fused spur (a switched outlet near the boiler) to make sure it is turned on and its fuse cartridge has not blown.
- Reset the boiler DIY safe
Once you have checked the supply, try resetting the boiler using the reset button or control knob as described in your user guide. Attempt a reset no more than two or three times. If F17 returns each time, a further reset will not solve the underlying problem.
- Call a qualified electrician to test the supply and wiring Gas Safe engineer
If the basic checks above do not resolve the fault, an electrician should attend to measure the actual mains voltage at the property and at the boiler's spur, and to inspect the wiring for loose connections or degraded cable. Resolving the electrical side first avoids unnecessary engineer call-outs.
- Arrange a Gas Safe registered engineer to inspect the boiler internally Gas Safe engineer
Once an electrician has confirmed the supply is stable and within spec, a Gas Safe engineer can safely open the boiler, check the internal fuse, inspect the wiring loom connections, and test the PCB. Never open the boiler casing yourself — there is live mains voltage inside, and the proximity of water makes this genuinely dangerous.
- PCB replacement if required Gas Safe engineer
If the Gas Safe engineer determines the PCB has failed or suffered surge damage, it will need to be replaced or sent for specialist repair. This is a skilled task requiring correct isolation, testing, and re-commissioning of the appliance.
Parts you may need
- Boiler internal fuse (typically 3A ceramic) · from £5
- Fused spur unit (if replacement needed) · from £15
- Replacement 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 £80–£300, depending on the underlying cause.
Frequently asked questions
Is F17 a gas fault or an electrical fault?
F17 is purely an electrical fault — it means the mains voltage reaching the boiler has dropped below the 170V minimum. There is no problem with the gas supply, the burner, or the heating circuit. That said, you will still need a Gas Safe engineer to inspect the boiler internally if the electrical supply checks out fine, because work inside the casing requires a competent and qualified person.
Can I fix the Glow-Worm F17 fault myself?
You can safely carry out a few basic checks: look for tripped breakers at the consumer unit, check that the fused spur is switched on, and attempt a boiler reset. Beyond that, the fault requires either a qualified electrician (to test and repair the supply or spur wiring) or a Gas Safe engineer (to inspect internal connections and the PCB). Do not open the boiler casing yourself.
How much does it cost to fix an F17 fault?
Most homeowners pay somewhere between £80 and £300 to resolve an F17 fault. A straightforward electrician visit to check the supply and replace a fused spur typically costs £80–£150. If the internal fuse needs replacing, that is a minor addition to an engineer's call-out fee. PCB replacement — which is only needed in a minority of cases — can cost £300–£500 including parts and labour, which is why it is important to rule out supply and wiring issues first before assuming the board has failed.
Why does my Glow-Worm boiler keep showing F17 after I reset it?
If F17 returns after every reset, the underlying supply problem has not been fixed. Possible reasons include an intermittent connection that worsens under load, a mains voltage that is persistently low in your area, or a damaged PCB that cannot accurately process the incoming supply. A repeat fault always warrants a proper electrical investigation rather than further resets — repeated resets on an under-voltage supply can cause additional wear on the boiler's components.