Baxi 800 Heat (range) Fault codes & diagnostics

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

Regular Natural Gas 13-30 kW 2017 onwards
GC Numbers 41-470-5041-470-5141-470-5241-470-5341-470-54
Download the Baxi 800 Heat (range) manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
16 High
Versions & GC numbers

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
41-470-50 HEAT 412 2017–present
41-470-51 HEAT 415 2017–present
41-470-52 HEAT 418 2017–present
41-470-53 HEAT 424 2017–present
41-470-54 HEAT 430 2017–present
Just want the most common Baxi 800 Heat (range) faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 16 documented codes

Tap any card for details

LED / flash-pattern codes
1 flash green

Your boiler has detected that it is heating up too quickly or getting too hot, usually because the water isn't moving through the system properly.

High DIY-safe £100-350

What to check first

  1. Check that your pump is running and hasn't seized
  2. Ensure all radiator valves are open to allow water flow
  3. Check that the system pressure is at the correct level (usually 1.0 to 1.5 bar)
  4. Bleed all radiators to remove trapped air pockets

Parts commonly replaced

  • Circulating Pump
  • Flow and Return Thermistors
  • Main Heat Exchanger (if scaled up)
Full explanation for 1 flash green
1 flash red

Your boiler has stopped heating because it is struggling to accurately measure the water temperature or the water isn't moving through the system correctly.

High DIY-safe £90-180

What to check first

  1. Check that all radiator valves and boiler isolation valves are fully open
  2. Ensure your thermostat is calling for heat and has working batteries
  3. Reset the boiler by turning the selector switch to the reset position for five seconds

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Return temperature sensor (NTC)
  • Circulation pump

Related codes

Full explanation for 1 flash red
2 flashes green

Your boiler has shut down because a safety device or external control has broken the electrical circuit, telling the system to stop running.

High Engineer only £90-180

Parts commonly replaced

  • User Interaction Controller
  • External thermostat wiring
  • Link wire
Full explanation for 2 flashes green
2 flashes red

Your boiler has shut down because it has detected it is getting too hot, likely due to a lack of water circulation or a blocked flue.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open (at least two or three)
  2. Check that the external system pump is running if accessible
  3. Turn the selector knob to the 'Reset' position for five seconds and release

Parts commonly replaced

  • Over-temperature Thermostat
  • External Circulating Pump
  • Heat Exchanger (de-scaling)
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 2 flashes red
3 flashes green

Your boiler has lost its flame while it was running, which means it has stopped providing heating and hot water for safety.

High DIY-safe £100-£350

What to check first

  1. Check that your gas emergency control valve is open and you have credit on your gas meter.
  2. Verify if other gas appliances in your home, such as a hob, are working correctly.
  3. Press the reset button or turn the selector switch to 'R' for five seconds to attempt a restart.

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Ignition lead

Related codes

Full explanation for 3 flashes green
3 flashes red

Your boiler is trying to start up but cannot light the gas or keep the flame burning, so it has shut down for safety.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances in your home, like a hob, are still working to confirm gas supply.
  2. If it is freezing outside, check that your external white plastic condensate pipe hasn't frozen.
  3. Press the Reset button or turn the selector knob to 'R' for five seconds and release.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Lead
  • Printed Circuit Board (PCB)
Full explanation for 3 flashes red
4 flashes green

Your boiler's internal components are having trouble talking to each other due to a wiring or electronic signal issue.

High Engineer only £120-£350

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Display board
Full explanation for 4 flashes green
4 flashes red

Your boiler's fan, which safely clears waste gases from the system, is not spinning at the correct speed or has stopped working entirely.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Venturi tube
Full explanation for 4 flashes red
5 flashes green

Your boiler’s internal computer has an electronic configuration error or a failed circuit board, meaning it cannot process instructions to run correctly.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Parameter Key
Full explanation for 5 flashes green
5 flashes red

Your boiler's internal computer has encountered a communication error or a software setting failure and cannot operate safely.

High Engineer only £180-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Parameter Key
Full explanation for 5 flashes red
6 flashes green

Your boiler's electronic control board has detected an internal communication error and cannot safely manage the heating process.

High Engineer only £250-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for 6 flashes green
6 flashes red

Your boiler has stopped working because of a communication failure or an electrical connection issue between its internal or external components.

High Engineer only £100-450

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • External controls interface
Full explanation for 6 flashes red
Mains LED flashing

Your boiler's control panel is not receiving a signal, which is preventing it from firing up to provide your heating and hot water.

High Engineer only £150-300

Parts commonly replaced

  • Interface PCB
  • Control PCB
  • Wiring harness
Full explanation for Mains LED flashing
Digital codes
Ignition Lockout

Your boiler has stopped working because it couldn't light the flame, so it has safely shut itself down to prevent gas from building up.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition lead
  • Printed Circuit Board (PCB)
Full explanation for Ignition Lockout
Lockout LED continuously on

Your boiler has stopped working because it failed to light the flame, which means you currently have no heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition lead
  • PCB
Full explanation for Lockout LED continuously on
Mains LED off

Your boiler has lost all power and is completely shut down, meaning it cannot provide any heating or hot water.

High DIY-safe £90-250

What to check first

  1. Check your home's consumer unit (fuse box) to ensure the boiler's circuit breaker hasn't tripped.
  2. Ensure the fused spur switch located near the boiler is turned on.
  3. Check that the fuse inside the fused spur switch has not blown.

Parts commonly replaced

  • Fused spur
  • PCB
  • Transformer
Full explanation for Mains LED off
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