LOCKOUT 3 FLASHES + DIAG 2 error code
LOCKOUT 3 FLASHES + DIAG 2 – Return thermistor disconnected or faulty
What does Keston Keston Celsius 25 and 25P condensing boilers LOCKOUT 3 FLASHES + DIAG 2 mean?
Keston identifies the Celsius 25/25P diagnostic pattern LOCKOUT 3 FLASHES + DIAG 2 as “Return thermistor disconnected or faulty”. This boiler has no numeric fault display, so the lamp flash count and illuminated diagnostic LED must be recorded exactly.
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 Keston Celsius 25 and 25P condensing boilers LOCKOUT 3 FLASHES + DIAG 2 error code indicates keston identifies the Celsius 25/25P diagnostic pattern LOCKOUT 3 FLASHES + DIAG 2 as “Return thermistor disconnected or faulty”. This boiler has no numeric fault display, so the lamp flash count and illuminated diagnostic LED must be recorded exactly. 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 fascia shows lockout 3 flashes + diag 2 and the boiler may remain off or locked out until the relevant demand, supply or fault condition is corrected.
Common causes
The official engineer table associates this pattern with return thermistor disconnected or faulty, involving the related supply, controls, pressure, thermistor, fan, ignition, flame, overheat, wiring or PCB 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
A qualified engineer should test the relevant thermistor, wiring and control-board connection.
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 LOCKOUT 3 FLASHES + DIAG 2 reappears, note when it happens and give that information to the service engineer.
Reset instructions
After the cause has been corrected, reset by turning the user control knob off and then on once. Do not repeatedly reset a recurring fault.
Applicable products and models
Keston Celsius 25 and 25P condensing boilersCelsius 25; Celsius 25P; KC25; KC25P2000 manual generationWhen to contact a professional
Contact a suitably qualified Gas Safe registered engineer when a lockout persists or returns, or when diagnosis requires internal measurements, wiring checks or replacement parts.
Frequently asked questions
What does Keston Keston Celsius 25 and 25P condensing boilers LOCKOUT 3 FLASHES + DIAG 2 mean?
Keston identifies the Celsius 25/25P diagnostic pattern LOCKOUT 3 FLASHES + DIAG 2 as “Return thermistor disconnected or faulty”. This boiler has no numeric fault display, so the lamp flash count and illuminated diagnostic LED must be recorded exactly.
Can I fix Keston Keston Celsius 25 and 25P condensing boilers LOCKOUT 3 FLASHES + DIAG 2 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 LOCKOUT 3 FLASHES + DIAG 2?
After the cause has been corrected, reset by turning the user control knob off and then on once. Do not repeatedly reset a recurring fault.
When should I arrange a repair?
Contact a suitably qualified Gas Safe registered engineer when a lockout persists or returns, or when diagnosis requires internal measurements, wiring checks or replacement parts.