- Boilers
- Tankless gas water heater
F04
Saunier Duval F04 on a gas water heater: ignition failure
Risk level
High
Professional inspection is recommended.
Urgency
High
Quick answer
F04 on the Saunier Duval Opalia F water heater is an ignition error: the burner will not light or goes out unexpectedly. Check the gas taps are open and reset with the switch; after three failed attempts, call a technician.
Safety first
Stop and check this first
- If you smell gas while the Opalia F shows F04, do not operate switches, plugs, doorbells or phones, avoid any flame, open doors and windows if you can, close the meter or main gas valve and the appliance gas tap if possible, warn neighbours without ringing, leave the building, keep others out and call the emergency services and the gas supplier emergency line from outside.
- With F04 on the Opalia F, if you smell fumes, a CO alarm sounds or anyone has a headache, dizziness or nausea, open doors and windows on your way out, switch the appliance off only if it is on your path, get into fresh air and call for urgent help from outside, because carbon monoxide has no smell.
Safe checks you can perform
- Check both gas taps
- Reset with the switch, three times at most
- Bear the gas type in mind
Leave these tasks to a qualified professional
- Do not attempt this professional task: Check gas supply and pressure
- Do not attempt this professional task: Diagnose ignition and gas valve
- Do not attempt this professional task: Review combustion on completion
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Check gas supply and pressure
What they doThe gas technician confirms the gas valve is open and measures the gas flow pressure with the heater running.
They must show youthe measured gas pressure against the data plate value.
In writingthe pressure value and whether the supply needed correcting.
Diagnose ignition and gas valve
What they doIf the electrodes do not light the burner, they inspect the gas valve control and the ignition and monitoring electrodes.
They must show youthe ignition and flame detection test.
In writingthe faulty part and the repair carried out.
Review combustion on completion
What they doBefore returning the Opalia to use, they check tightness and operation of the product and record the maintenance work.
They must show youthe heater lighting several times in a row without F04.
In writingthe tightness check and recorded work.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Check both gas taps
The user manual says to open the installation gas shut-off and the Opalia appliance gas tap if either is closed. Look at their position without dismantling anything; if you find them closed, open them before resetting F04.
Evidence: Saunier Duval Opalia F 12/1 LR y F 14/1 LR, instrucciones de funcionamiento 0020265481_04
Reset with the switch, three times at most
Turn the on and off switch to OFF and back to ON; the heater restarts. If F04 repeats after three manual ignition attempts, stop trying and call an authorised professional.
Evidence: Saunier Duval Opalia F 12/1 LR y F 14/1 LR, instrucciones de funcionamiento 0020265481_04
Bear the gas type in mind
On liquefied gas the Opalia goes to F04 after a single failed attempt, compared with three on natural gas. Check the data plate under the appliance to see whether your model is H, B or P and tell the technician.
Evidence: Saunier Duval Opalia F 12/1 LR y F 14/1 LR, instrucciones de funcionamiento 0020265481_04
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Saunier Duval
- Product type
- Tankless gas water heater
Known code variants
- F04
- F4
- Opalia F.04
- Opalia F04
- Saunier Duval F04
What it means
On the Opalia F 12/1 LR and F 14/1 LR, F04 appears when the burner cannot light or goes out unexpectedly. The user manual specifies that the heater locks out after three failed attempts on natural gas, or after one on liquefied gas. The installation manual separates two situations: an interrupted gas supply, which calls for checking the gas valve and flow pressure, and ignition electrodes failing to light the burner because of a gas valve control fault, which needs the service technician.
Warnings and stop conditions
- Criticaleditorial warning
If you smell gas while the Opalia F shows F04, do not operate switches, plugs, doorbells or phones, avoid any flame, open doors and windows if you can, close the meter or main gas valve and the appliance gas tap if possible, warn neighbours without ringing, leave the building, keep others out and call the emergency services and the gas supplier emergency line from outside.
Evidence: Saunier Duval Opalia F 12/1 LR y F 14/1 LR, instrucciones de funcionamiento 0020265481_04
- Criticaleditorial warning
With F04 on the Opalia F, if you smell fumes, a CO alarm sounds or anyone has a headache, dizziness or nausea, open doors and windows on your way out, switch the appliance off only if it is on your path, get into fresh air and call for urgent help from outside, because carbon monoxide has no smell.
Evidence: Saunier Duval Opalia F 12/1 LR y F 14/1 LR, instrucciones de funcionamiento 0020265481_04
Symptoms and causes
Probable causes · in order
- Gas tap closed or supply interruptedCommon
If the property gas shut-off or the appliance gas tap is closed, or the supply is interrupted, the burner cannot light.
Evidence: Saunier Duval Opalia F 12/1 LR y F 14/1 LR, instrucciones de instalación y mantenimiento 0020265480_08
- Gas flow pressure too low at the heaterPossible
The installation manual asks for the gas flow pressure to be checked when F04 shows; incorrect pressure prevents stable ignition.
Evidence: Saunier Duval Opalia F 12/1 LR y F 14/1 LR, instrucciones de instalación y mantenimiento 0020265480_08
- Gas valve control faultPossible
When the ignition electrodes cannot light the burner, the table points to a gas valve control fault.
Evidence: Saunier Duval Opalia F 12/1 LR y F 14/1 LR, instrucciones de instalación y mantenimiento 0020265480_08
Virtual technician
Does F04 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Saunier Duval · 2025 · manual
- 2
Saunier Duval Opalia F 12/1 LR y F 14/1 LR, instrucciones de funcionamiento 0020265481_04
Saunier Duval · 2020 · manual
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