Baxi 825 Combi Fault codes & diagnostics

6 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 25 kW 2019-present
GC Number 47-077-38
Download the Baxi 825 Combi manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
6 High
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GC number Appliance (register name) Produced
47-077-38 825 COMBI 2019–present
Just want the most common Baxi 825 Combi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 6 documented codes

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15

Your boiler's main internal computer has developed a communication fault and is unable to run the system safely.

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring Harness

Related codes

Full explanation for 15
28

Your boiler has detected a problem with the sensor that monitors the temperature of the exhaust gases, causing the system to shut down for safety.

High Engineer only £100-£180

Parts commonly replaced

  • Flue NTC Sensor
  • Connecting wiring harness

Related codes

Full explanation for 28
40

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning from your radiators.

High Engineer only £90-160

Parts commonly replaced

  • Return NTC thermistor
  • Connecting wiring harness

Related codes

Full explanation for 40
110

Your boiler has switched itself off because it has detected an overheating issue to keep the system safe.

High DIY-safe £90-250

What to check first

  1. Check that your radiator valves are open and not turned to zero.
  2. Ensure your boiler pressure is between 1 and 1.5 bar on the gauge.
  3. Perform a 'Reset' by turning the selector knob to the 'R' position for five seconds.

Parts commonly replaced

  • Safety Overheat Thermostat
  • Circulation Pump
  • Temperature Sensor (NTC)

Related codes

Full explanation for 110
125

Your boiler has detected that water is not moving through the system properly, which is causing it to shut down to prevent overheating.

High DIY-safe £120-£350

What to check first

  1. Check that your system pressure is between 1 and 1.5 bar on the front gauge.
  2. Ensure all radiator valves and isolation valves under the boiler are fully open.
  3. Bleed your radiators to remove any trapped air pockets.

Parts commonly replaced

  • Circulation Pump
  • Water Pressure Sensor
  • Automatic Air Vent

Related codes

Full explanation for 125
135

Your boiler’s main control board has encountered an internal electronic error or a loose connection, preventing the system from running safely.

High Engineer only £280-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring Harness

Related codes

Full explanation for 135
Status & diagnostic codes

81 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

1.0 Status Your boiler is currently performing an automatic internal check or initial setup process and is not experiencing an actual fault. 1.0.0 Diagnostic Your boiler's internal control system is struggling to 'talk' to its main computer, meaning it has paused operations as a safety precaution. 1.0.2 Diagnostic Your boiler has detected an internal configuration error that prevents it from starting correctly, meaning you currently have no heating or hot water. 1.1 Diagnostic Your boiler has detected a general safety issue that requires the system to shut down to prevent potential damage or danger. 1.2 Diagnostic Your boiler has detected a communication error or a configuration issue that prevents it from starting safely. 1.3 Diagnostic Your boiler has detected an internal safety irregularity that is preventing it from starting up correctly. 1.3.1 Diagnostic Your boiler is experiencing a communication error between its internal controls, meaning it has temporarily stopped working to prevent further faults. 1.4 Diagnostic Your boiler has reached its programmed end-of-life interval, meaning it is time to have a professional engineer perform a safety inspection and service. 1.4.1 Diagnostic Your boiler has reached the end of its programmed operational life and requires a professional service or system review to ensure it is still safe to run. 1.4.2 Diagnostic Your boiler has reached its programmed service limit and requires a routine professional inspection to ensure it continues to run safely and efficiently. 1.4.3 Diagnostic Your boiler has detected that a critical internal control component has reached the end of its programmed operational life and requires replacement by a professional. 2.1 Diagnostic Your boiler has detected a general internal fault that is preventing it from starting up correctly. 2.2 Diagnostic Your boiler has detected an issue with the electrical connection to its internal gas control system, preventing it from starting up safely. 2.2.1 Diagnostic Your boiler has detected an internal communication or software issue, meaning it cannot properly monitor its own temperature rise and has safely shut down to prevent damage. 2.2.2 Diagnostic Your boiler has detected an internal issue with its safety certification settings, which means it will stop working to ensure it continues to operate within legal safety limits. 2.2.3 Diagnostic Your boiler has detected an issue with the settings that control how it processes the gas supply, meaning it has safely shut itself down to prevent improper operation. 2.2.4 Diagnostic Your boiler has detected an internal configuration mismatch, meaning it cannot properly identify its own operating settings. 2.3 Diagnostic Your boiler is experiencing a communication error between its internal electronic parts, meaning it cannot safely operate or control its own functions. 2.3.1 Diagnostic Your boiler is experiencing a communication error between its internal electronic components, meaning it cannot properly coordinate its heating and hot water functions. 2.4 Diagnostic Your boiler has detected an issue with the water pressure or the safety mechanism that ensures water is flowing correctly, causing it to stop operating to prevent internal damage. 2.4.2 Diagnostic Your boiler is struggling to detect or maintain the correct water pressure and flow throughout its internal system. 2.4.3 Diagnostic Your boiler has detected a water leak inside its casing that needs to be inspected by a professional to prevent damage to internal electrical components. 2.5 Diagnostic Your boiler is struggling to ignite the flame reliably, which means the unit is currently preventing your heating and hot water from working to keep things safe. 3.1 Diagnostic Your boiler has detected an internal communication error and has safely shut down to prevent further issues. 3.2 Diagnostic Your boiler has detected an issue with the sensor that monitors the water pressure or the heat flow, causing it to shut down for safety reasons. 3.2.2 Diagnostic Your boiler is experiencing a complex internal communication error between its electronic components, meaning it cannot safely start or control your heating. 3.2.4 Diagnostic Your boiler has detected a blockage in the pipework that carries away waste water, which is preventing it from operating safely. 3.2.5 Diagnostic Your boiler has identified a internal communications failure between its main control boards, preventing it from operating correctly. 3.2.6 Diagnostic Your boiler has detected that the water inside the system is dirty or contaminated, which is triggering a safety sensor to protect the internal components from damage. 3.2.8 Diagnostic Your boiler has detected a blockage in the pipe that drains away waste water, causing it to shut down to prevent internal damage. 4.1 Diagnostic Your boiler has detected an issue with how it is communicating with its internal safety systems or controls, preventing it from starting up safely. 4.2 Diagnostic Your boiler has detected an issue with its installation setup, often caused by a communication error between the boiler and its external controls or settings. 4.2.1 Diagnostic Your boiler's internal computer has encountered a memory or software configuration error that is preventing it from starting up correctly. 4.2.2 Diagnostic Your boiler is displaying a service or setup notification indicating that the initial installation settings require attention or verification. 4.2.3 Diagnostic Your boiler is struggling to detect a stable gas supply, which prevents it from igniting safely. 4.2.4 Diagnostic Your boiler has detected a communication error between its internal electronic brains, causing it to shut down for safety. 4.2.5 Status Your boiler is having trouble detecting the correct water pressure or flow, which prevents it from heating your home or water safely. 4.2.6 Diagnostic Your boiler has detected an issue with the system's internal air or exhaust path, preventing it from operating safely. 4.2.7 Diagnostic Your boiler has identified a communication error with its internal control system that is preventing the heating from turning on. 4.3 Status Your boiler has detected that the water temperature has risen too high and it has shut down automatically to prevent damage. 4.4 Status Your boiler is struggling to detect water flow properly, which is preventing it from firing up to provide your heating or hot water. 4.4.1 Diagnostic Your boiler has detected an internal communication error between its main control components, causing it to shut down for safety. 5.1 Diagnostic Your boiler has detected a general internal fault that is preventing it from operating correctly. 6.1 Diagnostic Your boiler has detected a general internal fault that is preventing it from operating correctly. 6.1.6 Diagnostic Your boiler has detected an issue with the internal air or exhaust path that prevents it from firing safely. 6.2 Status Your boiler has detected an internal communication error between its internal components, meaning it cannot safely regulate its heating process. 6.2.1 Diagnostic Your boiler has stopped working because it has detected a problem with the internal air or flue system, meaning it is not safe to ignite. 6.2.2 Diagnostic Your boiler has detected an issue with the flow of gas or the ignition process that prevents it from starting up safely. 6.2.5 Status Your boiler has detected a failure in its internal communication or electronic control system, meaning it cannot safely operate. 6.2.6 Status Your boiler has detected a critical failure in the internal hardware communication, meaning it has shut down as a safety precaution and will not provide heating or hot water. 6.3 Status Your boiler is experiencing a communication error between its internal electronic control parts, which prevents it from operating correctly. 6.3.1 Diagnostic Your boiler is struggling to detect a stable gas supply and is currently performing a safety check to ensure it can ignite safely. 7.1 Diagnostic Your boiler has detected a general internal fault that is preventing it from operating correctly. 7.2 Status Your boiler has detected an internal communication error between its internal components, preventing it from starting up safely. 7.2.1 Diagnostic Your boiler has detected an issue with the gas supply or the combustion process, meaning it has safely shut down to prevent unsafe operation. 7.2.2 Diagnostic Your boiler has detected an issue with the internal electronics or communication between its main parts, which has caused it to stop working to prevent further damage. 7.3 Status Your boiler has detected a safety-related issue with its internal communication or monitoring systems, causing it to shut down to prevent damage. 7.3.1 Diagnostic Your boiler has detected an issue with the way it is managing its gas supply, which prevents it from operating safely. 7.4 Status Your boiler is experiencing a communication error between its internal sensors, which has caused it to stop working for your safety. 7.4.1 Diagnostic Your boiler has identified an internal communication error between its electronic components, meaning it cannot safely operate the heating or hot water. 7.4.2 Status Your boiler has detected an internal communication error and has safely shut down to prevent further issues. 7.5 Status Your boiler is experiencing a communication error between its internal electronic parts, meaning it has stopped working until the settings are correctly synchronised. 7.5.1 Diagnostic Your boiler has detected a communication error between its internal electronic brains, causing it to shut down for safety. 8.1 Diagnostic Your boiler has detected an internal issue with its electronic control system which is preventing it from operating correctly. 8.2 Status Your boiler has detected an internal communication error between its main control components, causing it to shut down for safety. 8.2.5 Status This code indicates a communication error between your boiler's internal controls, meaning the unit cannot safely manage its heating operations. 8.3 Status Your boiler has detected an issue with the sensor that monitors the water flow inside the system, preventing it from heating up correctly. 9.1 Diagnostic Your boiler has detected a general internal fault that is preventing it from operating correctly. 9.2 Diagnostic Your boiler has detected a general internal fault that prevents it from operating safely and needs a qualified professional to investigate. 9.2.1 Diagnostic Your boiler's computer is experiencing a temporary software confusion and cannot process its internal instructions correctly. 9.3 Status Your boiler has detected an internal communication error between its main control components, causing it to shut down for safety reasons. 9.3.1 Diagnostic Your boiler's computer is experiencing a software glitch or internal communication error, causing it to shut down to prevent incorrect operation. 9.3.2 Status Your boiler has detected an internal communication error between its main control components and has automatically shut down to prevent damage. 9.3.3 Diagnostic Your boiler has detected an internal communication failure between its main control components, causing it to shut down for safety. 9.3.4 Diagnostic Your boiler has detected an internal communication error between its main control components, causing it to shut down for safety. 9.4 Status Your boiler has detected an internal communication error between its main control boards, which prevents it from operating safely. 9.4.2 Status Your boiler has detected an internal communication error between its internal components, meaning it has safely shut itself down to prevent potential damage. 9.5 Status Your boiler is experiencing a communication failure where the internal circuit boards are unable to talk to each other properly. 10.1 Diagnostic Your boiler has detected an internal issue with its electrical communication system, causing it to shut down for safety. 11.1 Diagnostic Your boiler has detected an internal communication error between its main control boards, which has caused it to stop working for safety reasons. 20 Status Your boiler is currently in standby mode and is working exactly as it should.
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