31 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
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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 |
|---|---|---|
| 41-583-20 | Activ A Plus 16S | 2011–present |
| 41-583-21 | Activ A Plus 25S | 2011–present |
| 41-583-14 | Activ A 12OV | 2010–present |
| 41-583-15 | Activ A 15OV | 2010–present |
| 41-583-16 | Activ A 20OV | 2010–present |
| 41-583-17 | Activ A 25OV | 2010–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.
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Your boiler has stopped working as a safety precaution and will not restart, likely because it failed to light or detected an internal issue.
Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent damage.
Your boiler has detected that there is not enough water circulating inside the system to operate safely.
Your boiler has detected an issue with the internal ventilation system that removes exhaust gases, and it has safely shut itself off to prevent unsafe operation.
Your boiler has stopped heating because a temperature sensor is sending incorrect information, meaning the system doesn't know when the water is hot enough.
Your boiler is struggling to read the outdoor temperature, which means it may not be heating your home as efficiently as it should be.
Your boiler is struggling to read the outdoor temperature, which might cause your heating to run less efficiently or feel slightly out of sync with the weather.
Your boiler has detected that a sensor responsible for monitoring the temperature of the gases leaving your flue is not providing a correct reading, so it has shut down to ensure safety.
Your boiler has detected that the temperature of the exhaust gases leaving the unit is too high, so it has safely shut itself down to prevent damage.
Your boiler has failed to light the flame properly and has shut down for safety reasons.
Your boiler has detected that water is not moving through the system properly, which has caused it to shut down to prevent overheating.
Your boiler is struggling to circulate water properly, which is preventing it from heating your home or water safely.
Your boiler is confused because two of its internal temperature sensors are either plugged into the wrong spots or the internal water pump has been installed the wrong way around.
Your boiler has stopped working because it cannot accurately measure the temperature of the water circulating inside, meaning it has shut down as a precaution to prevent overheating.
Your boiler has detected that there is not enough water circulating inside the system to heat your home or provide hot water safely.
Safety lock-out (alternates with the word 'reset' on the LCD display)
Lack of burner ignition on safety lockout
Safety thermostat intervention lockout
General lockout
Faulty primary circuit (no water or absence of flow)
Faulty fan control system
Faulty c.h. temp. probe NTC flow
Faulty external temp. probe NTC
Faulty flue temp. probe NTC
Flue probe intervention lockout
Your boiler has stopped working because a sensor that measures the temperature of the water returning to the system is failing to send a signal, causing the boiler to shut down as a precaution.
Faulty pump or primary temperature above 105°C
None or too low water flow; Faulty pump (temp. difference between probes higher than 35° C)
Possible exchange of NTC probes (Flow or Return) or pump wrongly mounted (upside – down)
Faulty c.h. temp. probe NTC (Flow or Return)
Faulty primary circuit (no water or absence of flow)
BoilerFaultCodes