Common problems guide

The most common Saunier Duval Themaclassic F25E problems

The faults most likely to send a Saunier Duval Themaclassic F25E 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.
15 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
Red flashing light High DIY-safe

Your boiler has detected a general issue and has entered a safety lockout mode.

What to try first

Locate the On/Off switch on the boiler control panel.Turn the switch to the (0) position and wait for five seconds.Turn the switch back to the (I) position to attempt a system reset.

Est. cost
£100-250
Parts
Ignition electrode, Printed Circuit Board (PCB), Fan assembly
Full guide for Red flashing light
02
Flashing pressure High DIY-safe

Your boiler has detected that the water pressure level inside the heating system has dropped below the required amount to operate safely.

What to try first

Locate the filling loop, which is usually a silver braided hose with one or two small valves underneath the boiler.Slowly open the valves to allow mains water into the system until the pressure gauge needle reaches the green zone (typically 1.0 to 1.5 bar).Close the valves tightly to stop the flow of water and check the display to see if the flashing warning has cleared.

Est. cost
£80-150
Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for Flashing pressure
03
F1 High Engineer

Your boiler is attempting to start up, but it is unable to keep the flame alight once it ignites, so it safely shuts itself down.

Est. cost
£150-300
Parts
Flame rectification electrode, Gas valve, Ignition lead
Full guide for F1
04
F4 High Engineer

Your boiler is attempting to start its flame but is unable to stay lit, causing it to shut down for safety reasons.

Est. cost
£120-250
Parts
Ionisation electrode, Ignition lead, Printed Circuit Board (PCB)
Full guide for F4
05
E01 High Engineer

Your boiler is failing to light the flame properly, so it has turned itself off for safety until it is repaired.

Est. cost
£120-250
Parts
Ignition electrode, Gas valve, Printed Circuit Board (PCB)
Full guide for E01
06
E02 High Engineer

Your boiler is struggling to clear exhaust gases safely, so it has turned itself off to prevent any risk.

Est. cost
£120-250
Parts
Air pressure switch, Flue fan, Venture pipe
Full guide for E02
Parts most often replaced

on the Saunier Duval Themaclassic F25E

Ignition electrodePrinted Circuit Board (PCB)Fan assemblyPressure switchFilling loopPressure relief valve

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 Saunier Duval 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 15-code list for the Saunier Duval Themaclassic F25E

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →