BoilerFaultCodesWhat the manual says
Manufacturer manualMains (Green) ON, Boiler On (Yellow) OFF, Lockout (Red) Flashing 3 times every 2 seconds, DIAG LED 2 ON
From this model's manual
Verified against the Keston/Keston Celsius 25 installation & service manual.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register →
Boiler cover options
Avoid the next call-out bill
A cover plan would handle faults like Return thermistor not connected or faulty: no call-out fees, no surprise bills.
Approved partner
Get a quote →
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Affiliate links. We may earn a commission at no extra cost to you.
All 23 fault codes on the Celsius 25
Seeing a different code? Every code this model can display, in one place.
Boiler lost flame whilst runningBoiler off on frontCabinet overheatExternal controls not calling for heatF2Fan pcb faultyFan speed failureFlow OverheatFlow thermistor not connected or faultyFlue OverheatIgnition failureLockout lamp flashing 2 times every 2 secondsLockout lamp flashing 3 times every 2 secondsLockout lamp flashing 4 times every 2 secondsLockout lamp flashing 5 times every 2 secondsLockout lamp flashing 15 timesModulation LED 1 litModulation LED 2 litModulation LED 3 litNo power to boilerSpare sensor/wiring loom faultWater pressure low