Saunier Duval F.57 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.57 fault code mean?
The F.57 code indicates that your Saunier Duval boiler has detected a serious underlying fault and has automatically downgraded itself into a reduced 'degraded mode' as a safety precaution. F.57 is not itself the root cause — it is a secondary code triggered by one of three primary faults: F52 (mass flow sensor calibration failure), F53 (flame failed to stabilise after ignition), or F56 (air-to-gas ratio imbalance, carrying a risk of carbon monoxide production). When F.57 is active, heating and hot water output will be reduced or intermittent, and the boiler may lock out repeatedly even after manual resets. Identifying which of the three underlying codes triggered F.57 is the key first step — some boiler models store this in a fault history menu.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Blocked or faulty MFS (Mass Flow Sensor) filter Common
On electronic -B versions of the ThemaPlus and ThemaFast Condens ranges, the mass flow sensor sits beneath a venturi cap and contains a small filter. Over time this filter accumulates dust, condensate residue, and gas circuit debris. Once partially or fully blocked, the sensor reports flow readings that do not match what the control board expects, causing the board to log F53 (no flame stabilisation) or F52 (MFS calibration fault), which in turn triggers F.57.
- Low gas supply pressure Common
If the gas pressure reaching the boiler falls below around 20 mbar, the air-to-gas mixture becomes incorrect for stable combustion. This can result from a partially closed isolation valve, a faulty meter regulator, or a temporary low-pressure event on the local distribution network. The flame either fails to establish or cannot hold, triggering F53 or F56 and dropping the boiler into degraded mode.
- Faulty or sticking modulating gas valve / stepper motor Sometimes
The stepper motor adjusts the modulating gas valve to fine-tune gas flow to the burner. Wear, a loose wiring connection, or internal motor failure can cause the valve to open incorrectly, producing an unstable or uneven flame. The control board detects the instability and logs F53 or F56, leading to F.57. This fault typically requires gas component replacement by a Gas Safe engineer.
- Dirty or misaligned ignition or flame-detection electrodes Sometimes
Deposits of combustion residue on the ignition or ionisation electrodes can prevent the flame signal from being correctly detected, even when a flame is present. The control board interprets this as flame failure and records F53, which chains to F.57. Electrode cleaning or repositioning is a common part of a boiler service but must be carried out by a qualified engineer.
- Blocked air intake or restricted flue Sometimes
If the air supply to the burner is restricted — for example by a partially blocked flue terminal, a bird nest, or debris on the air intake — the combustion mixture is starved of oxygen. This produces an unstable or incomplete flame, which the board records as F53 or F56 (with CO risk) and initiates degraded mode via F.57.
- Faulty PCB (control board) Rare
In a minority of cases, the control board itself generates incorrect sensor readings due to a power surge, micro-voltage fluctuation, or ageing electronic components, causing it to spuriously log F52, F53, or F56 without any peripheral component actually being defective. This is diagnosed by a process of elimination after all other components have been checked.
How to fix it
- Note the displayed code and attempt a single boiler reset DIY safe
Before anything else, make a note of the F.57 code and the time it appeared. Press the reset button on your boiler (refer to your model's user guide for the exact location — it is typically a flame icon or a dedicated reset key). Allow the boiler to attempt a restart. If F.57 clears and the boiler runs normally, monitor it closely over the next 24 hours. If the code reappears, do not keep resetting repeatedly — two or three attempts are the safe maximum before escalating.
- Check your gas supply is working properly DIY safe
Confirm that other gas appliances in your home (such as a gas hob or gas fire) are working normally. If none of your gas appliances have a supply, the issue is with the incoming gas rather than the boiler — contact your gas supplier or call the National Gas Emergency Service on 0800 111 999 immediately. Also confirm that the boiler's isolation valve (the handle inline with the gas pipe behind the boiler) is fully open — the handle should be parallel to the pipe.
- Check the fault history on the boiler display (if available) DIY safe
Some Saunier Duval models allow you to scroll through a fault log in the settings or diagnostic menu. Look for whether the root code stored is F52, F53, or F56 — this information is very useful for the engineer, as it narrows down which component to focus on first. Note down any codes you find and pass them to your heating engineer when you call.
- Arrange for a Gas Safe registered engineer to inspect the MFS sensor and filter Gas Safe engineer
The most frequent cause of F.57 — especially on ThemaPlus and ThemaFast Condens -B models — is a blocked filter inside the mass flow sensor venturi. An engineer will remove, inspect, clean, or replace the MFS unit and verify that the sensor's readings fall within the expected calibration range. This is not a DIY task as it involves opening the gas circuit.
- Have the engineer check gas pressure and combustion performance Gas Safe engineer
A Gas Safe engineer will connect a gas pressure gauge to verify that the supply pressure is within specification (typically around 20 mbar at the boiler). They will also carry out a combustion analysis with a flue gas analyser to identify whether an air-to-gas imbalance (F56) is present, which carries a carbon monoxide risk and must be resolved before the boiler is returned to service.
- Have the engineer inspect the gas valve, stepper motor, and electrodes Gas Safe engineer
If the MFS and gas pressure are both satisfactory, the engineer will move on to the modulating gas valve and stepper motor, checking for correct operation and wiring integrity, and will inspect the ignition and ionisation electrodes for contamination or misalignment. Faulty components will be replaced with genuine Saunier Duval parts and the system recalibrated.
- If all components test correctly, consider PCB replacement or further diagnostics Gas Safe engineer
Should all peripheral components prove serviceable, the engineer may use the Saunier Duval diagnostic kit to interrogate the control board more deeply. A board generating false fault signals will need replacement. The engineer will also recalibrate the system fully before signing off the repair.
Parts you may need
- MFS (Mass Flow Sensor) · from £85
- Modulating gas valve · from £120
- Gas valve stepper motor · from £75
- Ignition / ionisation electrode set · from £35
- PCB / control board · from £220
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
What is the difference between F.57 and F52, F53, or F56 on a Saunier Duval boiler?
F52, F53, and F56 are the primary fault codes that identify the actual problem — respectively a mass flow sensor calibration error, failure to stabilise a flame after ignition, and an air-to-gas ratio imbalance. F.57 is a secondary code that appears when one of those three faults has already been logged and the boiler has switched itself into a reduced 'degraded mode' as a safety measure. Think of F.57 as the boiler saying 'I've already hit a serious fault and I'm running in a limited capacity.' Fixing F.57 means identifying and resolving whichever primary code triggered it.
Can I reset a Saunier Duval F.57 fault myself?
Yes, a single reset attempt is safe and worthwhile — press the reset button and see whether the boiler runs normally afterwards. However, if F.57 returns after one or two resets, repeatedly resetting the boiler will not fix the underlying problem and may mask important diagnostic information. At that point, you should stop resetting and contact a Gas Safe registered engineer to investigate the root cause (F52, F53, or F56).
How much does it cost to fix a Saunier Duval F.57 fault in the UK?
Most people with an F.57 fault pay somewhere between £120 and £380 all in, depending on which component is at fault. Cleaning or replacing the MFS sensor tends to be at the lower end (around £100–£200), while a gas valve or stepper motor replacement typically falls in the £180–£280 range. In the relatively rare scenario where the PCB needs replacing, costs can rise to £350–£500 or beyond, depending on parts availability for your specific model — particularly on older boilers where some parts may need to be sourced from specialist suppliers.
Is F.57 dangerous — could it cause carbon monoxide poisoning?
F.57 itself triggers when the boiler has already detected a potentially dangerous condition and restricted its own operation. In particular, the F56 sub-fault (which can cause F.57) relates directly to an air-to-gas ratio imbalance that carries a carbon monoxide risk. For this reason, you should not attempt to bypass or repeatedly reset the boiler if F.57 keeps returning. Ensure you have a working, in-date carbon monoxide alarm fitted near the boiler, and arrange for a Gas Safe engineer to carry out a proper combustion check before the appliance is returned to normal use.