Common problems guide

The most common Protherm 40-50 CI problems

The faults most likely to send a Protherm 40-50 CI 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.
7 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
neon 1 not lit High DIY-safe

Your boiler is not receiving any electrical power, which could be due to a tripped switch or an internal fuse failure.

What to try first

Check if your home's main fuse box has tripped a circuit breaker.Ensure the boiler's isolation switch (fused spur) is turned on.Try replacing the 3-amp fuse in the external boiler switch if you are comfortable doing so.

Est. cost
£90-350
Parts
Internal PCB fuse, Printed Circuit Board (PCB), External Fused Spur
Full guide for neon 1 not lit
02
neon 2 not lit High DIY-safe

Your boiler has shut down because it has detected it is getting too hot, likely triggered by a safety switch to prevent damage.

What to try first

Turn off the power supply to the boiler.Allow the boiler to cool down completely for at least 20 minutes.Locate the reset button (usually behind the small plastic cover or on the control panel) and press it firmly.Restart the boiler and check if the radiators are bled of air.

Est. cost
£90-180
Parts
Overheat Thermostat, Limit Stat, Circulation Pump
Full guide for neon 2 not lit
03
neon 3 not lit High Engineer

Your boiler has detected an issue with how it moves air and exhaust gases, which has caused it to stop running for safety.

Est. cost
£120-280
Parts
Fan assembly, Air pressure switch, Venturi tube
Full guide for neon 3 not lit
04
neon 4 not lit High Engineer

Your boiler has failed to light the gas flame or cannot detect that it is lit, so it has shut down as a safety precaution.

Est. cost
£120-300
Parts
Ignition Electrode, Gas Valve, Flame Sensing Probe
Full guide for neon 4 not lit
05
Main burner not operating (all neons lit) High Engineer

Your boiler has detected a communication failure between its main computer and the firing controls, causing all the lights to stay on and the heating to stop working.

Est. cost
£180-350
Parts
Gas Valve Multi-functional Control, Wiring loom/Harness, Printed Circuit Board (PCB)
Full guide for Main burner not operating (all neons lit)
06
F1 High Engineer

Your boiler has stopped working because it is not receiving the necessary electrical power or an internal electrical safety fuse has blown.

Est. cost
£120-250
Parts
PCB fuse, Main printed circuit board (PCB), Wiring harness
Full guide for F1
Parts most often replaced

on the Protherm 40-50 CI

Internal PCB fusePrinted Circuit Board (PCB)External Fused SpurOverheat ThermostatLimit StatCirculation Pump

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 Protherm 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 7-code list for the Protherm 40-50 CI

Related guides

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