BoilerFaultCodes Lockout lamp flashing 2 times every 2 seconds
Ignition failure
Keston Celsius 25 Fault codes & diagnostics
23 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
System Natural Gas 25 kW 2006-2015 Discontinued
GC Number 41-930-06
Replaced by: Keston System 30
Download the Keston Celsius 25 manual
PDF
This boiler uses LED flash patterns
Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →
Severity at a glance
Each cell = one fault code. Hover to identify. 1 High
22 Unrated
Just want the most common Keston Celsius 25 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes
All 23 documented codes
Tap any card for details
LED / flash-pattern codes
Lockout lamp flashing 3 times every 2 seconds
Flow or return thermistor open circuit
Lockout lamp flashing 4 times every 2 seconds
Water pressure switch open circuit
Lockout lamp flashing 5 times every 2 seconds
Fan fault
Lockout lamp flashing 15 times
Control PCB fault
Digital codes
Boiler lost flame whilst running
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing twice every 2 seconds, DIAG LED 2 ON
Boiler off on front
Mains (Green) Rapid Flash, Boiler On (Yellow) OFF, Lockout (Red) OFF
Cabinet overheat
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing once every 2 seconds, DIAG LED 2 ON
External controls not calling for heat
Mains (Green) Slow Flash, Boiler On (Yellow) OFF, Lockout (Red) OFF
F2 Your boiler has lit correctly but the flame has gone out unexpectedly, causing the system to shut down for safety.
Your boiler has lit correctly but the flame has gone out unexpectedly, causing the system to shut down for safety.
What to check first
- Check if other gas appliances like your oven or hob are working to ensure gas is reaching the property
- Check if your gas meter has credit (if using a pre-payment meter)
- Reset the boiler by pressing the reset button for five seconds
Parts commonly replaced
- Flame sensing electrode
- Gas valve
- Ignition lead
- Condensate trap
Related codes
Showing 11–20 of 23
Fan pcb faulty
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing constantly
Fan speed failure
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing 5 times every 2 seconds
Flow Overheat
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing once every 2 seconds, DIAG LED 1 ON
Flow thermistor not connected or faulty
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing 3 times every 2 seconds, DIAG LED 1 ON
Flue Overheat
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing once every 2 seconds, DIAG LED 3 ON
Ignition failure
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing twice every 2 seconds, DIAG LED 1 ON
Modulation LED 1 lit
Flow overheat thermostat open circuit
Modulation LED 2 lit
Cabinet thermal fuse/overheat
Modulation LED 3 lit
Flue overheat thermostat activation
No power to boiler
Mains (Green) OFF, Boiler On (Yellow) OFF, Lockout (Red) OFF
Showing 21–23 of 23
Return thermistor not connected or faulty
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing 3 times every 2 seconds, DIAG LED 2 ON
Spare sensor/wiring loom fault
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing once every 2 seconds, DIAG LED 4 ON
Water pressure low
Mains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing 4 times every 2 seconds