What F70 means
F70 is a configuration fault rather than a sign that something has worn out. Vaillant's ecoTEC plus installation and maintenance instructions (document 0020244997) list it as "Fault: Invalid Device Specific Number", with the cause given as an incorrect or missing Device Specific Number, or an incorrect or missing coding resistor. The 2021 instructions for the open-vent ecoTEC plus (document 0020238195) call it an "Invalid product code (DSN)".
The DSN is the boiler's identity as far as its electronics are concerned. It tells the control board which model it is fitted to, so it can use the right settings for that boiler's output. The same 2021 manual lists "Code (DSN) Specific product code" as one of the entries on the boiler's data plate, which is where an engineer reads the correct number from. Without a valid DSN, the board will not run the boiler, because it cannot be sure it is operating within that model's limits.
Why F70 usually appears after a repair
The 2021 manual is specific about the most common trigger: the display and the PCB were "replaced at same time" and the Device Specific Number was not reset. In other words, both of the parts that hold the boiler's settings were swapped in one visit, and the number was not set again afterwards. The second cause the manual lists is a wrong or missing output coding resistor, a small plug-in part that tells the board the boiler's output.
So if F70 appeared the day a new board or display went in, that timing is the strongest clue you will get. If it appeared on a boiler nobody has touched, tell the engineer that too, because it changes where they will start looking.
| Situation | Most likely explanation | Who to call |
|---|---|---|
| F70 straight after a new PCB and display were fitted | DSN not entered on the new parts | The engineer who did the repair |
| F70 after a board was fitted from a different boiler or a non-matching spare | Wrong or missing coding resistor, or the wrong part | The engineer who did the repair |
| F70 on a boiler with no recent work | Coding resistor or electronics problem that needs testing | Gas Safe registered engineer |
Which Vaillant boilers use F70
We found F70 with the same meaning in all four Vaillant fault tables we checked:
- ecoTEC plus VU and VUW ../5-5 regular, system and combi models (combi 825 to 838), document 0020244997.
- ecoTEC pro 24 and 28 combis (VUW 246/5-3 and 286/5-3), document 0020244995.
- ecoTEC plus VU ../6-5 OVZ open-vent regular boilers, document 0020238195 (2021).
- ecoTEC plus VU 246/6-5 OVZ LPG boiler, document 0020239015.
What you can do
There is very little a homeowner can do to clear F70 directly, because the fix is a setting inside the boiler's service menu. What you can do is make sure the right person comes back with the right information.
- Note the date and what was replaced at the last visit. If you have a job sheet or invoice, find it.
- Check the boiler's fused spur is switched on, and try switching it off for a minute and back on. This will not change the DSN, but it rules out a one-off start-up glitch.
- Press reset once as your manual describes. If F70 stays, do not keep trying.
- If a repair was done recently, contact that engineer first and explain that the boiler is showing F70. Setting the DSN should have been part of fitting the parts.
- Have the boiler's model name and serial number ready, and a photo of the data plate if you can see it without removing any panels.
Do not ask anyone who is not Gas Safe registered to work on the board or enter codes in the service menu. The control board runs the gas valve and ignition, and the boiler needs to be checked properly once it is set up.
How an engineer fixes F70
Vaillant's measure for F70 in the ecoTEC plus instructions is direct: if the display and PCB have been replaced, change the Device Specific Number under D.93. The 2021 manual lists d.93 as "Setting the product code", adjustable from 0 to 99. The engineer reads the correct value from the data plate, enters it, and then confirms the boiler runs normally.
If the number is right and F70 remains, the engineer will look at the coding resistor, checking that one is fitted and that it matches your boiler's output. A replacement board that is not the right part for your model would also need to be swapped. Once the boiler is running, they should check its operation before leaving, as they would after any board replacement.
If you are having a board replaced for another fault, such as F63, Vaillant's EEPROM fault, it is worth asking the engineer beforehand whether they will be setting the DSN as part of the job.
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Small part or wiring repair, such as a coding resistor | Typically £100 to £180 |
| PCB replacement, fitted (if the wrong board was supplied) | Typically £250 to £550 |
Where F70 is the result of a repair that was not finished, ask the original engineer to return and complete it before you pay anyone else. Our boiler PCB replacement cost guide covers what a board replacement should include. You can find other ecoTEC electronics codes on our Vaillant fault codes page.
What to tell the engineer
Give them the full code, the date it first appeared, what was replaced at the last visit and by whom, and whether the boiler showed any other codes before the repair. If the original fault was a board problem, mention which code it was, since that tells them whether the new board has fully solved it once F70 is cleared.