THERMAL FUSE OPEN error code
THERMAL FUSE OPEN – Keston 50/60/80 fault
What does Keston Legacy Keston 50, 60 and 80 condensing boilers THERMAL FUSE OPEN mean?
A non-resettable thermal protection device has interrupted the control circuit.
The code identifies the condition detected by the product; it does not always identify the failed component on its own. Confirm the full code and exact model before ordering parts or attempting a reset.
The Keston Legacy Keston 50, 60 and 80 condensing boilers THERMAL FUSE OPEN error code indicates a non-resettable thermal protection device has interrupted the control circuit. This guide explains what the code means, the symptoms and likely causes, the checks that are safe to make, and when to stop and arrange an authorised repair.
Symptoms you may notice
The legacy boiler shows thermal fuse open or stops at the corresponding stage of its operating sequence. These models use amber, red and green lamps rather than a numeric fault display.
Common causes
The official Keston fault-finding flowchart associates this state with the related supply, thermostat, pressure-switch, fan, ignition, gas-control, flame-sensing, circulation or safety circuit.
Before you start troubleshooting
Use the display, rating label or product menu to confirm the exact model. Similar-looking codes can have different meanings across product generations and regions.
- Record the complete code before switching the equipment off.
- Check only user-accessible controls, filters and external connections described in the manual.
- Do not open gas, refrigerant, combustion or electrical compartments.
Troubleshooting steps
Do not bypass the device. A qualified engineer must identify the overheating cause and replace the approved component where required.
Can you fix this error code yourself?
The safest next step depends on the exact model and the cause behind the code. External checks may help, but a returning fault needs professional diagnosis.
A successful reset is not proof that the underlying issue is fixed. If THERMAL FUSE OPEN reappears, note when it happens and give that information to the service engineer.
Reset instructions
Record the lamp pattern and operating stage. Switch the boiler off, wait at least 20 seconds and try once only where the user instructions permit; do not keep resetting a recurring lockout.
Applicable products and models
Legacy Keston 50, 60 and 80 condensing boilersKeston 50; Keston 60; Keston 801996–2000 manual generationWhen to contact a professional
Contact a suitably qualified Gas Safe registered engineer when the state persists, lockout recurs, or diagnosis requires covers, instruments, wiring tests or component replacement.
Frequently asked questions
What does Keston Legacy Keston 50, 60 and 80 condensing boilers THERMAL FUSE OPEN mean?
A non-resettable thermal protection device has interrupted the control circuit.
Can I fix Keston Legacy Keston 50, 60 and 80 condensing boilers THERMAL FUSE OPEN myself?
Some external checks may be suitable for the user, but the result depends on the exact model and cause. Use the official manual and arrange service if the code returns.
Should I reset the product after code THERMAL FUSE OPEN?
Record the lamp pattern and operating stage. Switch the boiler off, wait at least 20 seconds and try once only where the user instructions permit; do not keep resetting a recurring lockout.
When should I arrange a repair?
Contact a suitably qualified Gas Safe registered engineer when the state persists, lockout recurs, or diagnosis requires covers, instruments, wiring tests or component replacement.