Baxi 400 Heat Fault codes & diagnostics

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

Heat Natural Gas 2017 onwards
GC Numbers 41-470-5141-470-5241-470-53
Download the Baxi 400 Heat 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.
9 High 1 Medium
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
41-470-51 HEAT 415 2017–present
41-470-52 HEAT 418 2017–present
41-470-53 HEAT 424 2017–present
Just want the most common Baxi 400 Heat faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 10 documented codes

Tap any card for details

1 green flash

Your boiler has likely detected that it is getting too hot and has paused operation as a safety precaution.

High DIY-safe £90-250

What to check first

  1. Check that your external pump is running and hasn't seized.
  2. Ensure all radiator valves are fully open to allow water to circulate.
  3. Check that the system pressure is between 1 and 1.5 bar.
  4. Reset the boiler by turning the selector switch to 'R' for five seconds.

Parts commonly replaced

  • Circulation pump
  • NTC Temperature Sensor
  • Heat exchanger (flushing service)

Related codes

Full explanation for 1 green flash
1 red flash

Your boiler has stopped working because it cannot get an accurate temperature reading from its internal sensors.

High Engineer only £100-180

Parts commonly replaced

  • Flow NTC thermistor
  • Return NTC thermistor
  • Wiring harness

Related codes

Full explanation for 1 red flash
2 green flashes

Your boiler has paused because a safety switch or an external control has told it to stop running temporarily.

High DIY-safe £80-160

What to check first

  1. Check if your thermostat or programmer is set to 'ON' and calling for heat.
  2. Ensure any external bypass valves or safety switches haven't been accidentally tripped.
  3. Try resetting the boiler by turning the selector switch to 'R' for five seconds and then back to 'On'.

Parts commonly replaced

  • External thermostat
  • Wiring harness
  • Control PCB

Related codes

Full explanation for 2 green flashes
2 red flashes

Your boiler has switched itself off because it has become too hot internally, likely due to a circulation problem.

High DIY-safe £100-350

What to check first

  1. Ensure all radiator valves are fully open to allow water to flow.
  2. Check that your system pressure is between 1 and 1.5 bar on the gauge.
  3. Press the reset button or turn the selector switch to 'R' for 5 seconds.

Parts commonly replaced

  • Circulating pump
  • Overheat thermostat
  • Heat exchanger

Related codes

Full explanation for 2 red flashes
3 green flashes

Your boiler is struggling to keep its flame lit, which is causing it to stop providing heating and hot water as a safety precaution.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure gas is reaching the property
  2. If it is freezing outside, check that the white plastic condensate pipe leading outdoors hasn't frozen
  3. Reset the boiler by turning the selector knob to the 'R' position for five seconds

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Spark Generator

Related codes

Full explanation for 3 green flashes
3 red flashes

Your boiler has failed to light after several attempts and has locked itself for safety.

High DIY-safe £100-£250

What to check first

  1. Check that your credit or prepaid gas meter has sufficient funds.
  2. Ensure other gas appliances, such as a hob, are receiving gas.
  3. Turn the selector switch to the reset position for five seconds, then turn it back to your desired setting.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Spark generator

Related codes

Full explanation for 3 red flashes
4 green flashes

Your boiler is having trouble communicating with the external controls or its internal components, meaning it might not know when to turn on or off.

Medium DIY-safe £90-250

What to check first

  1. Check if your thermostat or programmer has power or fresh batteries.
  2. Ensure your thermostat is set to a temperature higher than the current room temperature.
  3. Reset the boiler by turning the selector switch to the 'R' position for at least five seconds.

Parts commonly replaced

  • Room thermostat
  • Receiver unit
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 4 green flashes
4 red flashes

Your boiler has stopped working because the fan that helps remove waste gases is either stuck or not communicating with the control board.

High Engineer only £250-450

Parts commonly replaced

  • Flue Fan
  • PCB (Main Control Board)
  • Wiring Harness

Related codes

Full explanation for 4 red flashes
5 green flashes

Your boiler has encountered a configuration error where its internal settings have become confused or corrupted.

High Engineer only £100-£350

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • External Parameter Stick

Related codes

Full explanation for 5 green flashes
5 red flashes

Your boiler has encountered an internal error and has locked itself out for safety, meaning it will not provide any heating or hot water until reset.

High DIY-safe £100-£350

What to check first

  1. Turn the selector switch to the 'R' (Reset) position and hold it for at least five seconds.
  2. Release the switch and wait for the boiler to attempt to restart.
  3. Check that your gas pre-payment meter (if applicable) has credit and that the main gas lever is open.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 5 red flashes
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