Baxi Solo 3 40 PF Fault codes & diagnostics

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

Regular Natural Gas 11.7 kW 1999-2007 Discontinued

Replaced by: Baxi Solo 3 HE

This boiler uses indicator light combinations

Note which lights are on, off, or flashing and at what speed. Match the combination to the cards below. Do not reset until you have noted the pattern. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
2 Emergency 22 High 11 Low

All 35 documented codes

Tap any card for details

Indicator light inside box lit

Your boiler has detected that the electrical wiring coming into the unit is connected the wrong way around, which is a safety risk.

Emergency Engineer only £90-160

Parts commonly replaced

  • External wiring connection
  • Control PCB
Overheat indicator light

Your boiler has detected that it got too hot or lost power, so it has automatically turned itself off to stay safe and needs to be reset.

High DIY-safe £100-250

What to check first

  1. Turn the boiler settings dial to the 'Reset' position (or press the reset button if applicable).
  2. Wait for the boiler to cycle through its ignition sequence.
  3. If the light stays on, check if your heating system has enough water pressure or if a radiator valve is stuck closed.

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • Printed Circuit Board (PCB)
Boiler On light

Your boiler is working normally and is currently receiving a signal to provide heating or hot water.

Low DIY-safe

What to check first

  1. Check that your thermostat is set to the desired temperature
  2. Ensure your programmer or timer is set to an 'on' period
  3. Verify that the gas supply is turned on if the light is on but the burner has not ignited

Related codes

Boiler On light on

Your boiler is receiving power and has been told to turn on by your thermostat, so it should be operating normally.

Low DIY-safe

What to check first

  1. Check that your room thermostat is set to a temperature higher than the current room temperature
  2. Ensure your programmer or timer is set to an 'ON' period
  3. Check if the gas supply is turned on at the meter

Related codes

Burner On

Your boiler is signalling that it is trying to light, but it is encountering a blockage or electrical issue that prevents the fuel from flowing correctly.

High Engineer only £120-250

Parts commonly replaced

  • Gas valve
  • Burner manifold
  • Ignition lead
Burner On light off

Your boiler is likely shutting down because it has reached its maximum safe temperature or has a fault within its internal control board.

High Engineer only £180-350

Parts commonly replaced

  • Main PCB
  • Potentiometer
  • Temperature Sensor
E01

Your boiler has failed to light because it cannot detect a flame, meaning your heating and hot water will currently be turned off.

High Engineer only £120-220

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Printed Circuit Board

Related codes

Showing 11–20 of 35
E03

Your boiler has automatically turned itself off because it has become too hot to operate safely.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • PCB
E24

Your boiler has detected an issue with the component that controls the gas supply, meaning it cannot safely ignite to provide heating or hot water.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • PCB
E26

Your boiler is struggling to clear exhaust gases out through the flue, so it has safely shut itself down to prevent any unsafe buildup.

High Engineer only £180-350

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • Wiring harness
E26 186

Your boiler has detected an issue with the internal airflow sensors, which is preventing it from safely starting up and producing heat.

High Engineer only £150-250

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venturi tube

Related codes

E26 219

Your boiler is failing to detect the airflow needed for safe operation, so it has shut down to prevent any risk.

High Engineer only £150-250

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venturi assembly
E26 221

Your boiler has detected an issue with the internal air flow system, which prevents it from igniting safely to start heating your home.

High Engineer only £150-250

Parts commonly replaced

  • Air pressure switch
  • Flue fan assembly
  • Venturi tubes
Fan On

Your boiler has stopped working because it cannot confirm that the internal fan, which clears exhaust gases, is receiving power.

High Engineer only £120-250

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • Wiring harness / electrical connection
Fan/Flue LED Green

Your boiler is correctly sensing that the fan is pushing air through the system, which is a normal part of the startup sequence.

Low DIY-safe £0-120

What to check first

  1. Check that the external flue pipe is not blocked by debris or nests
  2. Ensure the electricity supply to the boiler is stable
  3. If the light stays on but the boiler doesn't fire, try resetting the power at the fused spur

Parts commonly replaced

  • None (Normal operation)
  • Air Pressure Switch
  • Fan
Lockout

Your boiler has automatically shut down to protect itself, likely because it got too hot or experienced a temporary glitch.

High DIY-safe £90-180

What to check first

  1. Locate the main control knob on the front panel.
  2. Turn the knob to the OFF position and wait at least 10 seconds.
  3. Turn the knob back to the ON position to attempt a reset.

Parts commonly replaced

  • Overheat Thermostat
  • PCB
  • Pump

Related codes

Lockout light

Your boiler has shut down because it has detected an unsafe temperature or a component failure in the pilot light system.

High Engineer only £120-250

Parts commonly replaced

  • Overheat Thermostat
  • Thermistor Sensor
  • Gas Control Valve
  • Pilot Solenoid
Showing 21–30 of 35
Lockout light on

Your boiler has likely stopped working because it reached a temperature that is too high, causing it to shut down for safety.

High DIY-safe £90-180

What to check first

  1. Turn the control knob to the OFF position
  2. Wait for at least 10 seconds
  3. Turn the control knob back to the ON position and check if the boiler fires up

Parts commonly replaced

  • Overheat Thermostat
  • Pump
  • Thermistor
Overheat

Your boiler has detected that it is running too hot and has automatically switched itself off as a safety precaution to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • PCB
  • System bypass valve
Overheat (Red LED)

Your boiler has detected that it is running too hot and has automatically shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • PCB
Overheat light

Your boiler has detected that it got too hot and has automatically shut itself down to stay safe.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • PCB
Overheat Neon

Your boiler has automatically shut itself off because it reached a dangerously high temperature to prevent damage or safety hazards.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • PCB
Overheat Neon Illumination

Your boiler has become too hot and has automatically switched itself off for safety to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • PCB
Peripheral / Pump LED Green

Your boiler is receiving a signal from your thermostat to turn on, which is a normal part of the heating process.

Low DIY-safe

What to check first

  1. Check that your room thermostat is set to the desired temperature
  2. Ensure your programmer or timer is set to 'ON' or 'AUTO'
  3. Wait a few minutes for the boiler to complete its start-up sequence
Pilot On

Your boiler is struggling to keep its small pilot flame lit, which prevents the main heating system from starting up.

High Engineer only £120-220

Parts commonly replaced

  • Thermocouple
  • Pilot assembly
  • Gas valve

Related codes

Pilot On light on

Your boiler is failing to light its small internal starter flame, which is preventing the main burner from turning on to heat your home.

High Engineer only £120-250

Parts commonly replaced

  • Pilot Injector
  • Ignition Electrode
  • HT Lead
  • Gas Valve
Showing 31–35 of 35
Power LED Green

Your boiler is receiving electrical power and is ready to operate normally.

Low DIY-safe

What to check first

  1. Check that your room thermostat or programmer is calling for heat
  2. Ensure the boiler thermostat knob is turned to the desired temperature setting
  3. Check that the gas supply is turned on
Red neon

Your boiler has switched itself off because it has reached an unsafe temperature and needs to cool down and be reset.

High DIY-safe £90-180

What to check first

  1. Switch off the electricity supply to the boiler for a few seconds
  2. Turn the electricity supply back on
  3. Turn the boiler thermostat knob fully anticlockwise to the 'OFF' position
  4. Wait 10 seconds, then turn the knob back to your desired temperature setting

Parts commonly replaced

  • Overheat Thermostat
  • Circulating Pump
  • Heat Exchanger (Cleaning)
Red neon illuminated

Your boiler has detected that it is getting too hot and has automatically switched itself off to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • System filter
  • PCB