Boiler
E17
The E17 error code on a Glow-worm boiler indicates a flow temperature sensor fault — specifically that the boiler is detecting an abnormally high temperature on the primary circuit. This is a safety-triggered lockout and should be taken seriously.
What does this error mean?
Your Glow-worm boiler continuously monitors the temperature of water flowing through the heat exchanger. When the flow temperature sensor detects a reading that exceeds safe operating limits — or sends a signal the boiler’s PCB cannot interpret — it triggers E17 and shuts the boiler down to prevent damage or danger. In short, the boiler thinks it’s overheating, whether or not it physically is.
Est. repair cost
£150–£300 for NTC sensor repair; £300–£600+ for heat exchanger or PCB fault
🔍 Likely causes
🛠️ Step-by-step fix
Check the pressure gauge on the boiler front panel — it should read between 1 and 1.5 bar when cold. If it’s below 1 bar, repressurise the system using the filling loop (consult your boiler manual for instructions specific to your Glow-worm model).
Once pressure is correct, press and hold the reset button (usually marked with a flame or ‘Reset’ symbol) for 3–5 seconds. Allow the boiler 2–3 minutes to attempt a full restart.
If the boiler fires up but E17 returns within minutes, check all radiators are heating evenly. Cold radiators at the top suggest air locks — bleed each radiator using a radiator key, starting from the ground floor and working upward.
After bleeding, recheck the system pressure and repressurise if it has dropped below 1 bar, then attempt another reset.
If the fault persists after these steps, do not attempt further DIY. The fault is likely an internal component — NTC sensor, pump, heat exchanger, or PCB — and requires a Gas Safe registered engineer.
📞 When to call a professional
If E17 returns after a reset and repressurisaton, or if you smell gas, hear unusual noises, or the boiler feels excessively hot to the touch, stop and call a Gas Safe registered engineer immediately. Tell them: ‘My Glow-worm boiler is showing an E17 lockout — I suspect a flow temperature sensor fault or overheating issue.’ Do not attempt to open the boiler casing or touch internal components — this is illegal without Gas Safe registration and dangerous.
⚖️ Repair or replace?
An NTC sensor replacement typically costs £150–£300 including parts and labour from a Gas Safe engineer. If the heat exchanger is blocked or the PCB is at fault, costs can rise to £300–£600+. If your Glow-worm boiler is over 10–12 years old and facing a repair over £400, replacement with a modern A-rated combi boiler (typically £1,800–£2,800 fully installed in the UK) is usually the more cost-effective long-term decision.