Common problems guide

The most common Baxi Solo 3 40 PF problems

The faults most likely to send a Baxi Solo 3 40 PF into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
35 documented codes
6 most common
4 DIY-safe checks
2 engineer needed

The 6 most common faults

01
Overheat indicator light High DIY-safe

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.

What to try first

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

Est. cost
£100-250
Parts
Overheat thermostat, Pump, Printed Circuit Board (PCB)
Full guide for Overheat indicator light
02
Lockout High DIY-safe

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

What to try first

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

Est. cost
£90-180
Parts
Overheat Thermostat, PCB, Pump
Full guide for Lockout
03
Lockout light on High DIY-safe

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

What to try first

Turn the control knob to the OFF positionWait for at least 10 secondsTurn the control knob back to the ON position and check if the boiler fires up

Est. cost
£90-180
Parts
Overheat Thermostat, Pump, Thermistor
Full guide for Lockout light on
04
Red neon High DIY-safe

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

What to try first

Switch off the electricity supply to the boiler for a few secondsTurn the electricity supply back onTurn the boiler thermostat knob fully anticlockwise to the 'OFF' positionWait 10 seconds, then turn the knob back to your desired temperature setting

Est. cost
£90-180
Parts
Overheat Thermostat, Circulating Pump, Heat Exchanger (Cleaning)
Full guide for Red neon
05
Red neon illuminated High Engineer

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

Est. cost
£120-250
Parts
Overheat thermostat, Pump, System filter
Full guide for Red neon illuminated
06
Overheat (Red LED) High Engineer

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

Est. cost
£120-250
Parts
Overheat thermostat, Pump, PCB
Full guide for Overheat (Red LED)

on the Baxi Solo 3 40 PF

Overheat thermostatPumpPrinted Circuit Board (PCB)Overheat ThermostatPCBThermistor

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Baxi shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 35-code list for the Baxi Solo 3 40 PF