Vaillant F.70 fault codewhat it means and how to fix it
The boiler does not know which model it is. The device specific number is missing or wrong, which nearly always follows a board or display replacement.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.70
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Has the boiler had parts fitted recently?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Yes, a board or display was changed in the last few weeks
This is the explanation in the overwhelming majority of cases. The work is nearly complete and one configuration step remains.
Contact the engineer who carried out the repair. Setting the DSN takes minutes and should not be a new charge.
A used or reconditioned part was fitted
A second-hand board arrives carrying another appliance's number, so it has to be reconfigured for yours before it will run.
Make sure whoever fitted it has the documentation to set the DSN correctly for your model.
No, nothing has been touched
Less common, and it suggests the stored number has been corrupted or the coding resistor circuit has failed.
Book a Vaillant trained engineer, since resolving this needs access to the commissioning menu.
What F.70 means
F.70 means the Device Specific Number, which Vaillant abbreviates to DSN, is incorrect, missing, or does not match the coding resistor fitted to the appliance. The DSN is how an ecoTEC identifies itself: it tells the electronics which model and output they are installed in, and therefore which operating parameters to apply. That number is held between the display and the mainboard, and it is cross-checked against a physical coding resistor.
Replace one of those and the number carries over from the other. Replace the display and the mainboard at the same time and there is nothing left holding it, so the DSN has to be entered deliberately during commissioning. Missing that step produces F.70 immediately. It is a configuration fault, not a broken component, which is why the fix is usually a return visit rather than another part.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Display and mainboard replaced together | With both parts new there is no surviving record of the number, and it must be set by hand afterwards. | Common |
| DSN not entered at commissioning | The replacement work was done correctly but the configuration step at the end was overlooked. | Common |
| Wrong or missing coding resistor | The resistor that confirms the model was not transferred to the new part, or one for a different output was fitted. | Occasional |
| Spare part intended for another model | A component supplied against the wrong appliance reference will not agree with what the boiler expects to see. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£0–£180
Nothing where it follows recent work that should have included the configuration. Otherwise a chargeable visit of £80 to £180 to set the DSN correctly.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.63Get it checkedVaillant reports an EEPROM error, meaning the boiler cannot reliably read its own stored settings. The printed circuit board is at fault and needs replacing.Related: electronics and power
- F.64Get it checkedAn electronics or sensor fault: the board cannot make sense of a safety-relevant sensor signal. Vaillant's causes are a shorted flow or return sensor or a defective board.Related: electronics and power
- F.65Get it checkedThe electronics have got too hot. Vaillant say either something outside the boiler is heating the board, or the board itself is faulty; the boiler stops until it cools.Related: electronics and power
- F.67Get it checkedThe flame signal the board is receiving is not believable. Vaillant treat F.67 as an electronics fault in the flame-monitoring circuit rather than a real flame problem.Related: electronics and power
- F.97Get it checkedVaillant's main PCB has failed the check it runs on itself. The table gives one cause, "main PCB defective", and one remedy: replace the PCB.Related: electronics and power
- F.195Get it checkedA 2022 ecoTEC plus code: the boiler has measured a significant undervoltage in its mains supply. F.196 is the overvoltage twin. Vaillant's first instruction is to check the mains, not the boiler.Related: electronics and power
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.70
What is a DSN?
A Device Specific Number — a short code identifying exactly which ecoTEC model and output the electronics are fitted to. Once it is set, the boiler applies the right parameters for that appliance. Without it, the electronics have no idea what they are driving.
Can I set the DSN myself?
No. It lives in the installer menu behind an access code, and it requires the correct number for your specific appliance from Vaillant documentation. Entering a wrong value configures the boiler as a different model, which is worse than the fault you started with.
Should I have to pay for this?
If it followed a repair you have already paid for, no. Setting the DSN is part of completing that work properly. A reputable firm will treat it as finishing the job. If nothing has been touched and the fault arose on its own, it is a chargeable visit.
Will the boiler run at all like this?
No, and that is deliberate. Rather than operate on parameters it cannot verify, the appliance stops. The upside is that the fault is entirely recoverable — nothing has been damaged and no part is necessarily needed.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer support portal
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.