Guide
Boiler Fault Codes: When to Stop and Call a Professional
A UK-focused guide to knowing when a boiler fault has moved beyond safe user checks.
First separate an emergency from a routine fault. A code needs the exact model and full display; it cannot by itself establish a safe reset or identify a part to replace.
Last updated: 22 September 2026
Gas or carbon-monoxide emergency
- If you smell gas, suspect a gas leak or your carbon-monoxide alarm sounds, stop troubleshooting. Move outside and call the mains-gas emergency service from a safe place: 0800 111 999 in Great Britain or 0800 002 001 in Northern Ireland. For LPG, contact your supplier’s emergency service.
- Do not smoke, use flames or operate electrical switches where gas is suspected. Follow the emergency operator’s instructions and do not go back inside until advised it is safe.
- If carbon-monoxide exposure is suspected, seek urgent medical help. Do not wait for a routine boiler appointment.
Arrange qualified help for a recurring fault
- The complete code returns after the limited user action allowed in your exact manual, or no user action is specified.
- The heating system repeatedly loses pressure, shows an unexplained leak or will not remain within its specified range.
- You suspect the appliance is unsafe, or the next step would involve the casing, internal wiring, burner, flue or gas controls. Stop using it and arrange appropriate help.
- For a gas boiler, choose a Gas Safe registered engineer with the correct qualifications for that appliance and work. Check the ID card, including the work categories on the back.
Information to have ready
- The model and serial number from accessible paperwork or the rating label, without removing covers.
- A photograph of the entire display if it is safe to take one, including punctuation, prefixes and alternating numbers.
- When the problem occurs, the pressure reading if relevant and any user action already taken.
- Warranty details and any applicable official safety notice. If renting, report the fault to the landlord or managing agent.
Do not repeatedly reset a boiler or use another model’s filling instructions. A reset can clear a display without correcting its cause. The linked code pages state their evidence limits; an unconfirmed definition needs the applicable manual or manufacturer support before any diagnosis is made.
Browse related appliance hubs
Related brand and appliance hubs
Ideal Boiler hub
Open the full Ideal boiler code list connected to this guide.
Vaillant Boiler hub
Open the full Vaillant boiler code list connected to this guide.
Baxi Boiler hub
Open the full Baxi boiler code list connected to this guide.
Worcester Bosch Boiler hub
Open the full Worcester Bosch boiler code list connected to this guide.
High-priority boiler fault codes
Baxi Boiler E128
Frequent flame loss when the full code is E128
Baxi Boiler E28
Flue-temperature NTC circuit fault in the applicable Baxi table
Ideal Boiler F9
PCB fault on the documented Instinct2 models
Vaillant Boiler F22
Low water pressure
Vaillant Boiler F23
Excessive flow-to-return temperature difference
Vaillant Boiler F24
Boiler temperature rising too quickly
Vaillant Boiler F25
Flue-gas temperature too high
Vaillant Boiler F27
Flame signal detected with the burner off
Ignition codes
Vaillant Boiler F27
Flame signal detected with the burner off
Vaillant Boiler F28
No flame established during ignition
Baxi Boiler 133
Gas supply or ignition problem when the full code is E133
Ideal Boiler L2
Flame loss or ignition lockout
Ideal Boiler L6
False-flame lockout
Vaillant Boiler F54
Gas supply fault associated with F28 or F29
Flame-loss codes
Vaillant boiler faults needing cautious checks
Vaillant Boiler F22
Low water pressure
Vaillant Boiler F23
Excessive flow-to-return temperature difference
Vaillant Boiler F24
Boiler temperature rising too quickly
Vaillant Boiler F25
Flue-gas temperature too high
Vaillant Boiler F27
Flame signal detected with the burner off
Vaillant Boiler F28
No flame established during ignition
Vaillant Boiler F29
Flame lost during operation and re-ignition unsuccessful
Vaillant Boiler F32
Fan fault
Baxi boiler faults needing cautious checks
Baxi Boiler E128
Frequent flame loss when the full code is E128
Baxi Boiler E28
Flue-temperature NTC circuit fault in the applicable Baxi table
Baxi Boiler 117
High system pressure when the full display is E117
Baxi Boiler 118
Low system pressure when the full code is E118
Baxi Boiler 125
Circulation fault when the full code is E125
Baxi Boiler 130
Flue thermostat trip when the full code is E130
Baxi Boiler 133
Gas supply or ignition problem when the full code is E133
Baxi Boiler E1
Meaning unconfirmed; exact model evidence required
Frequently asked questions
Is every boiler fault code an emergency?+
No. Follow the exact model instructions for routine faults, but a gas smell, suspected gas leak or carbon-monoxide alarm requires the emergency route above. Do not wait for a normal service booking.
Can I reset the boiler once?+
Only if the exact user manual permits it for the displayed condition and there are no safety concerns. There is no universal reset allowance. If the fault returns, stop repeating the procedure and obtain help.
Is any heating engineer suitable for a gas boiler?+
For gas work, use a Gas Safe registered engineer qualified for the particular appliance and work. Check the relevant categories on the engineer’s ID card.
Sources and editorial limits
Sources support the explanations and checks on this page. Follow the instructions for your exact model; these sources do not confirm the cause or condition of an individual appliance.
Read our editorial policy and source methodology.
- HSE: Gas safety for home ownersGas work requires a Gas Safe registered engineer competent for the appliance and work. An unsafe gas appliance must not be used.Checked 22 September 2026
- National Gas: Emergency contactsGreat Britain gas and carbon-monoxide emergency number and precautions; this is separate from booking a repair.Checked 22 September 2026
- HSENI: Carbon monoxideNorthern Ireland gas emergency number and advice to stay outside and seek urgent medical advice if exposure is suspected.Checked 22 September 2026
- Worcester Bosch: Repeated pressure lossSupports visible leak observations, possible hidden pipework loss and contacting the installer or manufacturer. These possibilities do not confirm a failed component.Checked 22 September 2026
- HSE: Natural gas and LPG emergency routesDistinguishes the natural-gas emergency service from contacting the LPG supplier. The homeowner guidance above governs routine domestic gas work.Checked 22 September 2026