Baxi
The Baxi 818 System 2 is a wall-hung condensing system boiler available in 18, 24, and 30 kW outputs, featuring 94% seasonal efficiency and an ErP A-rating for heating.
The Baxi 818 System 2 is a wall-hung condensing system boiler designed for domestic heating installations in the UK. This appliance, part of the Baxi 800 System 2 family, is available in multiple output variants, including 18 kW, 24 kW, and 30 kW. It supports both natural gas and LPG fuel types, offering flexibility for various property types. This page provides comprehensive technical specifications, fault codes, and installation guidance for heating engineers and homeowners researching the model.
Stick a QR tag on the case, scan it on every service and give your customer a permanent, shareable service history they can pull up from any phone.
Permanent, shareable service history
One QR scan on the boiler surfaces every previous fault, part change and visit — no paperwork to dig out.
Free AI fault diagnostics, built in
Every tag you fit unlocks the AI diagnostic tool for that exact model — paste a fault code, get a ranked cause list.
Your name on the boiler forever
When the next service or breakdown comes around the homeowner scans and sees your number first.
First 12 BoilerTags are free
Tag every install and service you do from now on — the subscription only kicks in once you scale past your first dozen.
Diagnosing a fault on a Baxi 800 System 2 24 right now?
Try the free AI fault diagnostics tool — no signup needed.
Full technical specifications extracted from the manufacturer brochure. Empty sections are hidden.
Bottom connections, as shown in Fig.3 and Fig.4
Treatment of water in domestic hot water central heating systems (BS 7593)
The Baxi 818 System 2 is a robust condensing system boiler, engineered for efficiency and reliability in UK domestic heating applications. With a seasonal efficiency of 94% and an ErP heating rating of A, the appliance meets current energy performance standards. It features a wide range of heating outputs, from a minimum of 4.8 kW up to a maximum of 32.4 kW, accommodating diverse property sizes. The boiler is designed for wall-hung installation, measuring 700mm in height, 395mm in width, and 285mm in depth, with a dry weight of 29 kg. Its compliance with Boiler Plus and OpenTherm compatibility allows for integration with advanced control systems, including weather and load compensation, optimising system performance and user comfort. The unit operates with a maximum heating circuit pressure of 2.5 bar and includes a 7-litre expansion vessel. NOx emissions are rated Class 6, indicating low environmental impact.
Every fault code listed in the manufacturer brochure, with likely causes and engineer actions. Each code has a permalink — paste a URL like https://www.boilertag.com/boilers/baxi/800-system-2-24#fault-a.00 .34 to jump straight to the entry.
| Code | Meaning | Details |
|---|---|---|
| A.00 .34 | Outdoor temperature sensor expected but not detected Show causes & actionsLikely causes
Engineer action Connect the outdoor sensor | Details |
| A.02 .06 | Low water pressure Show causes & actionsEngineer action Check the system pressure and restore it Check the pressure of the expansion vessel Check for any system / boiler leaks | Details |
| A.02 .18 | OBD error Show causes & actionsLikely causes
Engineer action Re-enter the CN1 and CN2 values, check the information shown on the data plate | Details |
| A.02 .36 | Functional device disconnected Show causes & actionsLikely causes
Engineer action Check the connection and the connectors Faulty SCB, replace the board | Details |
| A.02 .37 | Passive functional device disconnected Show causes & actionsLikely causes
Engineer action Check the connection and the connectors Faulty SCB, replace the board | Details |
| A.02 .45 | Connection error Show causes & actionsLikely causes
Engineer action Carry out the auto-detect function | Details |
| A.02 .46 | Device priority error Show causes & actionsLikely causes
Engineer action Carry out the auto-detect function | Details |
| A.02 .48 | Unit function configuration error Show causes & actionsLikely causes
Engineer action Carry out the auto-detect function | Details |
| A.02 .49 | Failed node initialisation Show causes & actionsLikely causes
Engineer action Carry out the auto-detect function | Details |
| A.02 .54 | Open Therm bus power supply error Show causes & actionsEngineer action Replace the CU-GH board | Details |
| A.02 .55 | Incorrect or missing serial number Show causes & actionsEngineer action Replace the CU-GH board | Details |
| A.02 .76 | Internal memory reserved for full customisation of settings. No further changes can be made Show causes & actionsLikely causes
Engineer action Re-enter the values CN1 and CN2 Replace the CU-GH board | Details |
| H.00 .42 | Pressure sensor open/faulty Show causes & actionsLikely causes
Engineer action No action required | Details |
| H.01 .00 | Temporary communication failure between gas valve and boiler PCB. Show causes & actionsLikely causes
Engineer action Configure CN1/CN2 Replace the main PCB | Details |
| H.01 .05 | Maximum temperature difference value between flow and return reached. Show causes & actionsLikely causes
Engineer action Check the boiler/installation circulation Activate a manual venting cycle Check the installation pressure Check the cleanliness of the exchanger Check the operation of the temperature sensors Check the connection of the temperature sensors | Details |
| H.01 .08 | Flow temperature increase in heating mode too quick. Temporary stoppage of 10 minutes. Show causes & actionsLikely causes
Engineer action Check the boiler/installation circulation Activate a manual venting cycle Check the installation pressure Check the cleanliness of the exchanger Check the operation of the temperature sensors Check the temperature sensor connection | Details |
| H.01 .14 | Maximum flow temperature value reached. Show causes & actionsLikely causes
Engineer action Check the boiler/installation circulation Activate a manual venting cycle | Details |
| H.01 .18 | No water circulation (temporary). Show causes & actionsLikely causes
Engineer action Check the installation pressure Activate a manual venting cycle Check the operation of the pump Check the boiler/installation circulation Check the operation of the temperature sensors Check the temperature sensor connection | Details |
| H.01 .21 | Flow temperature increase in domestic water mode too quick. Temporary stoppage of 10 minutes. Show causes & actionsLikely causes
Engineer action Check the installation pressure Activate a manual venting cycle Check the operation of the pump Check the boiler/installation circulation Check the operation of the temperature sensors Check the connection of the temperature sensors | Details |
| H.02 .00 | Reset in progress | — |
| H.02 .02 | Waiting for configuration settings to be entered (CN1,CN2). Show causes & actionsLikely causes
Engineer action Configure CN1/CN2 | Details |
| H.02 .03 | Configuration settings (CN1, CN2) not entered correctly. Show causes & actionsEngineer action Check configuration CN1/CN2 Configure CN1/CN2 correctly | Details |
| H.02 .04 | PCB settings cannot be read. Show causes & actionsLikely causes
Engineer action Configure CN1/CN2 Replace the main PCB | Details |
| H.02 .05 | Setting memory not compatible with the boiler PCB type | — |
| H.02 .07 | Low pressure in heating circuit (permanently) Show causes & actionsEngineer action Check the system pressure and restore it Check the pressure of the expansion vessel Check for any system / boiler leaks | Details |
| H.02 .09 | Partial stoppage of the boiler Show causes & actionsLikely causes
| Details |
| H.02 .10 | Complete stoppage of the boiler Show causes & actionsLikely causes
| Details |
| H.02 .12 | Opening of the control unit input signal contact from the external device | — |
| H.02 .38 | No water hardness Show causes & actionsEngineer action Not possible | Details |
| H.02 .70 | External unit heat recovery test failed Show causes & actionsLikely causes
| Details |
| H.03 .00 | No identification data for boiler safety device. Show causes & actionsLikely causes
Engineer action Replace the main PCB | Details |
| H.03 .01 | Communication failure in comfort circuit (internal fault in boiler PCB). Show causes & actionsLikely causes
Engineer action Replace the main PCB | Details |
| H.03 .02 | Temporary flame loss Show causes & actionsLikely causes
Engineer action Check the electrode connection and wiring Check the condition of the electrode Check the gas supply pressure Check the gas valve calibration Check the pipes and the terminal | Details |
| H.03 .05 | Power supply voltage too low Show causes & actionsEngineer action Check the mains voltage | Details |
| H.03 .17 | Periodic safety check in progress | — |
| H.03 .54 | Temporary flame loss Shutdown due to the power supply voltage being too low Show causes & actionsLikely causes
Engineer action Check the electrode electrical connections Check the condition of the electrode Check the gas inlet pressure Check the gas valve calibration Check the air intake and flue gas exhaust terminal Check the power supply voltage | Details |
| E.00 .04Lockout | Return temperature sensor disconnected Show causes & actionsLikely causes
Engineer action Check the operation of the temperature sensor Check the connection of the sensor/PCB | Details |
| E.00 .05Lockout | Return temperature sensor short circuited Show causes & actionsLikely causes
Engineer action Check the operation of the sensor Check the sensor/PCB connection | Details |
| E.00 .06Lockout | Return sensor expected but not detected | — |
| E.00 .07Lockout | The difference in return temperature is too great Show causes & actionsLikely causes
Engineer action Check the operation of the sensor Check the sensor/PCB connection | Details |
| E.00 .16Lockout | DHW tank temperature sensor not connected Show causes & actionsLikely causes
Engineer action Check the operation of the sensor Check the sensor/PCB connection | Details |
| E.00 .17Lockout | DHW tank temperature sensor short-circuited | — |
| E.00 .20Lockout | The flue gas temperature sensor has short-circuited or measured a temperature below the range | — |
| E.00 .21Lockout | The flue gas temperature sensor has short-circuited or measured a temperature above the range | — |
| E.01 .04Lockout | Flame loss detected five times in 24 hours (with burner on) Show causes & actionsLikely causes
Engineer action Check the gas supply pressure Check the gas valve calibration Check the air intake and flue gas exhaust terminal Check the power supply voltage | Details |
| E.01 .12Lockout | Temperature measured by return sensor greater than flow temperature Show causes & actionsLikely causes
Engineer action Check that the sensors are positioned the correct way round Check that the flow sensor is in the correct position Check the return temperature in the boiler Check the operation of the sensors | Details |
| E.01 .17Lockout | No water circulation (permanent) Show causes & actionsLikely causes
Engineer action Check the installation pressure Activate a manual venting cycle Check the operation of the pump Check the boiler/installation circulation Check the operation of the temperature sensors Check the connection of the temperature sensors | Details |
| E.01 .20Lockout | Maximum flue gas temperature reached Show causes & actionsLikely causes
Engineer action Check the cleanliness of the exchanger | Details |
| E.02 .13Lockout | Stoppage input for the device external environ-ment control unit | — |
| E.02 .15Lockout | External PCB communication timeout | — |
| E.02 .17Lockout | Permanent communication failure between gas valve and boiler PCB Show causes & actionsLikely causes
Engineer action Check for any electromagnetic interference Replace the main PCB | Details |
| E.02 .35Lockout | Critical safety device disconnected Show causes & actionsLikely causes
Engineer action Carry out the auto-detect function | Details |
| E.02 .47Lockout | Connection to external device unsuccessful Show causes & actionsLikely causes
Engineer action Possible cancellation, carry out the auto-detect function or re-enter the CN code | Details |
| E.04 .00Lockout | Level 5 safety settings incorrect Show causes & actionsLikely causes
Engineer action Replace the PCB | Details |
| E.04 .01Lockout | Flow temperature sensor short circuited Show causes & actionsLikely causes
Engineer action Check the sensor/PCB connection Check the operation of the sensor | Details |
| E.04 .02Lockout | Flow temperature sensor disconnected Show causes & actionsLikely causes
Engineer action Check the sensor/PCB connection Check the operation of the sensor | Details |
| E.04 .03Lockout | Maximum flow temperature exceeded Show causes & actionsLikely causes
Engineer action Check the boiler/installation circulation Activate a manual venting cycle Check the operation of the sensors | Details |
| E.04 .07Lockout | Difference between the temperature values de-tected by flow sensors 1 and 2 | — |
| E.04 .08Lockout | Maximum safe temperature value reached Show causes & actionsLikely causes
Engineer action Check the pressure in the installation Switch on the manual degassing function Check that the pump is working Check the circulation in the boiler/installation Check the safety thermostat connection Check that the safety thermostat is working correctly | Details |
| E.04 .09Lockout | Difference between the temperature values de-tected by flue gas sensors 1 and 2 | — |
| E.04 .10Lockout | Burner failed to ignite after 4 attempts Show causes & actionsLikely causes
Engineer action Check the gas supply pressure Check the gas valve electrical connection Check the gas valve calibration Check the operation of the gas valve Check the electrode electrical connections Check the electrode condition Check the operation of the fan Check the condition of the flue gas exhaust (block-ages) | Details |
| E.04 .11Lockout | VPS gas valve test failed Show causes & actionsEngineer action Check the ground circuit Check the power supply voltage | Details |
| E.04 .12Lockout | Ignition failure for monitoring parasitic flame Show causes & actionsLikely causes
Engineer action Check the PCB-fan connection Replace the air-gas unit | Details |
| E.04 .13Lockout | Fan blade blocked | — |
| E.04 .14Lockout | The temperature difference between the set point and the burner varies more than 60 s compared to the GVC configuration | — |
| E.04 .15Lockout | Flue gas exhaust pipe blocked Show causes & actionsLikely causes
Engineer action Replace the main PCB | Details |
| E.04 .17Lockout | Fault in gas valve control circuit | — |
| E.04 .18Lockout | The flow temperature measured is lower than the minimum temperature defined by the GVC setting | — |
| E.04 .19Lockout | Mass flow rate sensor communication error | — |
| E.04 .20Lockout | Mass flow rate sensor maximum deviation | — |
| E.04 .21Lockout | Difference detected between the burner 1 and 2 temperature sensors Show causes & actionsEngineer action Not possible | Details |
| E.04 .23Lockout | Gas valve check valve internal stoppage | — |
| E.04 .24Lockout | Gas family not detected during the automatic de-tection function Show causes & actionsEngineer action Not possible | Details |
| E.04 .54Lockout | Fault in gas valve control circuit Show causes & actionsEngineer action Check the wiring on the gas valve Replace the gas valve or PCB | Details |
Patterns that group across the manufacturer's fault-code table. Each entry links to the full code listing further down the page.
Engineer: Check the operation of the temperature sensor Check the connection of the sensor/PCB
Triggered by fault code E.00 .04.
Engineer: Check the operation of the sensor Check the sensor/PCB connection
Triggered by fault code E.00 .05.
Triggered by fault code E.00 .06.
Engineer: Check the operation of the sensor Check the sensor/PCB connection
Triggered by fault code E.00 .07.
Engineer: Check the operation of the sensor Check the sensor/PCB connection
Triggered by fault code E.00 .16.
Diagnostic checklists from engineer-submitted Baxi faults, with step-by-step checks and safety notes.
The Baxi 800 boiler intermittently heating without a fault code can be attributed to several common issues, including low system pressure,…
The Baxi 100HE displaying an 'Ignition Lockout' fault, with no heating or hot water and no noises, indicates the boiler has failed to…
The Baxi 824 Combi 2 displaying fault code E.04.10 indicates that the burner has failed to ignite after four attempts.
Common questions from engineers and homeowners about the Baxi 800 System 2 24.
We hold the Baxi 800 System 2 24 manual — and every other UK boiler and heat pump brochure. Sign up and pull any model up from your phone, on the job.
Sign up to access brochuresTag it before you leave. Next scan shows every service, fault and part change — and the AI diagnostic tool is free on every tag you fit.
Specifications on this page are extracted from the manufacturer brochure. Always cross-check against the latest datasheet before commissioning or quoting. Last updated 28 June 2026.