Saunier Duval F.70 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.70 fault code mean?
The F.70 code points to an electronic identity or configuration problem with the boiler's main circuit board (PCB). Specifically, the boiler cannot find a valid Device Specific Number (DSN) — the software identity that tells the PCB exactly which Saunier Duval model it is installed in. Without this, the boiler refuses to operate as a safety measure. This is not a pressure fault, a sensor fault, or an ignition problem; it is purely an electronic configuration issue. Saunier Duval is part of the Vaillant Group and shares the same fault-code architecture: F.70 carries the same meaning across both brands. The code is displayed as two alternating numbers on the boiler's screen (the second number shows a decimal point). Critically, resetting the boiler will not clear this fault — the underlying configuration must be corrected by a Gas Safe registered engineer who can access the boiler's service menu and enter or verify the correct DSN.
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
- Unprogrammed or incorrectly matched replacement PCB Common
By far the most frequent trigger for F.70. The main PCB used across many Saunier Duval models is a generic board that must be programmed with a DSN specific to the exact boiler model before it can function. When an engineer fits a new board without completing this programming step — or fits a board intended for a different model — the boiler detects the mismatch at start-up and locks out immediately. The fix is for an engineer to access the service menu (typically via the diagnostic parameter D.93) and enter the correct DSN for that specific boiler.
- Faulty or disconnected e-Bus wiring Sometimes
The e-Bus is the internal data channel over which the PCB and the user interface (display/control panel) communicate. A loose connector, a chafed or broken wire, or a poor joint anywhere along this communication path can prevent the DSN from being read correctly, triggering F.70. This is more likely on older boilers where wiring looms have been disturbed during previous servicing.
- Moisture or corrosion on electronic components Sometimes
Water ingress into the electronics enclosure — from a nearby leak, excessive condensation, or a blocked condensate drain — can cause micro-short-circuits or oxidation on the PCB tracks and connector pins. The user interface board is particularly vulnerable. Corroded contacts disrupt e-Bus signal transmission and can cause the DSN to appear invalid or absent.
- Missing or incorrect coding resistor Rare
Saunier Duval boilers use a small gas-family coding resistor to identify the gas type the boiler is set up for. If this resistor is absent, short-circuited, or does not match the selected gas type, it can cause a configuration conflict that surfaces as F.70 (sometimes alongside an F.42 code). This is more likely after a component swap or if the boiler has been incorrectly converted between gas types.
- Electromagnetic interference disrupting e-Bus communication Rare
Strong electromagnetic fields from motors, transformers, or other appliances located close to the boiler's communication cables can corrupt the e-Bus signal intermittently. This is an uncommon cause but worth investigating if F.70 appears sporadically rather than constantly, and wiring and programming checks have drawn a blank.
How to fix it
- Do not attempt to repeatedly reset the boiler DIY safe
Unlike many boiler faults, F.70 will not clear with a standard reset because the root cause is a configuration or hardware issue, not a transient sensor trip. Pressing reset more than once or twice wastes time and will not help. Save yourself the frustration and move straight to the steps below.
- Check that the boiler has not recently had any parts replaced DIY safe
Think back over the last few weeks. Has an engineer visited and replaced the PCB, the display board, or any other electronic component? If so, F.70 almost certainly means the new board was not programmed with the correct DSN before the engineer left. Note down any parts that were changed and mention this when you call for help — it will speed up diagnosis.
- Visually inspect for obvious signs of water ingress or damage around the boiler casing DIY safe
With the boiler switched off at the mains, have a look around the outer casing and any visible wiring for signs of water staining, rust, or corrosion. Do not open the boiler casing or touch any internal components. If you spot evidence of a leak nearby, arrange for a plumber to fix it before the boiler is repaired, as the same moisture could damage a new PCB.
- Check that the gas supply to the property is working DIY safe
Confirm that other gas appliances (hob, fire) are working normally, and check that the gas isolation valve on the boiler's gas supply pipe is in the open position (handle in line with the pipe). F.70 is not a gas supply fault, but it is always worth ruling this out before calling an engineer.
- Call a Gas Safe registered engineer to diagnose and programme the DSN Gas Safe engineer
This is the essential step. A qualified engineer will access the boiler's service menu — typically by holding the relevant button combination — and navigate to the diagnostic parameter list (commonly parameter D.93) to check, enter, or correct the Device Specific Number for your exact boiler model. If the DSN has simply been wiped or never entered, this reprogramming alone (sometimes via a factory reset using parameter D.96 followed by re-entry of the correct DSN) can restore the boiler to full operation without any new parts being needed.
- Engineer to inspect and test all e-Bus wiring and connectors Gas Safe engineer
If the DSN appears correct in the service menu but F.70 persists, the engineer should methodically check every connector and cable in the e-Bus circuit — between the PCB, the display board, and any external controls — for corrosion, looseness, or damage. Communication cables should be routed away from any electrical motors or transformers that could cause interference.
- Engineer to replace the PCB if reprogramming does not resolve the fault Gas Safe engineer
If the DSN programming is confirmed correct but the fault continues, the main PCB itself is likely defective. The engineer will source a compatible replacement board and fit it, then immediately programme the correct DSN before closing up — failing to do this step would cause F.70 to reappear straight away. The engineer should verify full heating and hot water function before leaving.
Parts you may need
- Saunier Duval main PCB (model-specific) · from £260
- e-Bus communication cable/loom · from £35
- User interface/display board · from £120
- Gas-family coding resistor · from £15
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 £80–£400, depending on the underlying cause.
Frequently asked questions
Can I fix Saunier Duval F.70 myself by resetting the boiler?
Unfortunately not. F.70 is a configuration lockout, not a standard operational trip. The boiler has detected that its PCB either has no Device Specific Number assigned or has one that does not match the model. A simple reset cannot supply or correct that missing software identity. You need a Gas Safe registered engineer to access the boiler's service menu and enter the correct DSN — there is no homeowner workaround for this fault.
Why has F.70 appeared right after my boiler's PCB was replaced?
This is the classic scenario for F.70. The main circuit board used in many Saunier Duval models is a generic part that must be individually programmed with a Device Specific Number matching your exact boiler model. If the engineer who fitted the board did not complete this programming step — or used the wrong DSN — the boiler locks out with F.70 immediately. Contact the engineer who did the work; they should return to programme the board correctly at no additional charge. If they cannot resolve it, call another Gas Safe registered engineer who is familiar with Vaillant Group boilers.
How much does it cost to fix F.70 on a Saunier Duval boiler in the UK?
The cost depends entirely on what is actually wrong. If the PCB just needs its DSN programmed or corrected — which is the most common fix — you are typically looking at a call-out and labour charge of around £80 to £150 in total. If the PCB itself has failed and needs replacing, parts and labour together generally come to £250 to £400 for most models, though prices are higher in London and the South East. In rare cases where a genuine new PCB for an older or less common model is needed, costs can push above this range — worth discussing with your engineer before authorising the work.
Is F.70 the same fault on Vaillant boilers as on Saunier Duval?
Yes. Saunier Duval is owned by the Vaillant Group and the two brands share the same underlying boiler architecture and fault-code system. F.70 means an invalid or missing Device Specific Number on both brands. An engineer experienced with Vaillant boilers will be equally at home diagnosing and fixing F.70 on a Saunier Duval, which is useful to know if you are struggling to find a specialist in your area.