BoilerFaultCodes 16
Ignition Lockout
Keston Heat 55 Fault codes & diagnostics
36 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
Regular Natural Gas 55 kW 2013 onwards Discontinued
GC Number 41-930-41
Replaced by: Keston Heat 55 2
Download the Keston Heat 55 manual
PDF
Versions & GC numbers
The fault code is already on your display
Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →
Severity at a glance
Each cell = one fault code. Hover to identify. 36 Unrated
Which version do you have?
The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.
| GC number | Appliance (register name) | Produced |
|---|---|---|
| 41-930-41 | Heat 55 | 2013–present |
Appliance names and production years are taken from the UK Product Characteristics Database (the SAP/SEDBUK register). Where a GC number is also printed in this model's own manufacturer manual, both sources were checked to agree.
Just want the most common Keston Heat 55 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes
All 36 documented codes
Tap any card for details
74
OVERHEAT LOCKOUT
75
IGNITION LOCKOUT
76
FALSE FLAME LOCKOUT / ERROR 20
77
LOW WATER PRESSURE
78
FLAME LOSS
79
FAN FAULT
80
FLOW & FLUE THERMISTOR FAULT
81
RETURN THERMISTOR FAULT
82
OUTSIDE SENSOR FAULT
Showing 11–20 of 36
83
FLOW/RETURN REVERSED
84
CYLINDER THERMISTOR FAULT
85
EXTERNAL INTERLOCK OFF
86
BCC FAULT
87
NO CH BUT DHW OK
88
NO DHW BUT CH OK
89
NO DISPLAY
90
0-10V INTERFACE
BCC Fault
Ensure BCC fitted correctly, Replace BCC
Cylinder Thermistor Fault
Check Cylinder Thermistor Wiring, Check Cylinder Thmr (10K @ 25°C, 2K at 66°C)
Showing 21–30 of 36
Error 20
False Flame Lockout
Ext’ Interlock Off
External Interlock Operated, Reset Interlock
False Flame Lockout
Check Flame Sense Electrode, Check Flame Sense Electrode Wiring
Fan Fault
Check Fan Wiring, Check Fan
Flame Loss
Check Gas to Boiler, Check Condensate Pipe, Check Flue System, Check Flame Sensor / Lead
Flow/Flue Thermistor Fault
Check Flow Thermistor Wiring, Check Flow Thermistor (10K @ 25°C, 2K at 66°C)
Flow/Return Reversed
Check Pump, Check System Pipework
Flue Thermistor Fault
Check Flue Thermistor Wiring, Check Flue Thermistor (10k @ 25ºC, 2k @ 66ºC)
Ignition Lockout
Check Gas to Boiler, Check Condensate Pipe, Refer to Instructions, Reset Boiler
Low Mains Voltage
Less than 160V, Check Local Generator, Contact Electricity Provider
Showing 31–36 of 36
Low Water Pressure
Fill System to 1 Bar, Vent System
Outside Sensor Fault
Check Outside Sensor Wiring, Check Outside Sensor (10K @ 25°C, 2K at 66°C)
Overheat Lockout
Fill System to 1 Bar, Vent System, Check Pump Spins Free, Reset Boiler
PCB Fault
Ensure BCC fitted, Replace PCB
Return Thmr Fault
Check Return Thermistor Wiring, Check Return Thmr (10K @ 25°C, 2K at 66°C)
Too Many Resets
5 resets within 15 mins, Turn boiler power off and on