Vaillant F70 fault code: the boiler does not know which model it is

What does the Vaillant F70 fault code mean?

Vaillant F70 means the boiler's control board does not have a valid Device Specific Number, so it does not know which model it is fitted to and locks out instead of firing. You cannot fix this yourself. It needs an engineer, and if one has just replaced a part, call that firm back.

Vaillant's own consumer fault-code page describes F.70 as “The Device Specific Number (DSN) is incorrect, missing or does not match the coding resistor”. Its installation and servicing manuals put it more tersely, as “Invalid device specific number (DSN)”, and name the cause as “Display and PCB replaced at same time and DSN not reset, incorrect or missing output range coding resistance”. Both descriptions point at the same thing.

F.70 is on Vaillant's public fault-code list at vaillant.co.uk, written with a decimal point, which is why a search for “F70” often misses the manufacturer's own page.

Here is why the code exists. Every Vaillant circuit board is the same physical part across a lot of different boilers, so on installation it has to be told which appliance it is sitting in. That instruction is the Device Specific Number, and the boiler cross-checks it against a coding resistor inside the casing. If the number is missing, wrong, or does not match that resistor, the board has no safe idea of the correct output, so it shuts down rather than guess.

Vaillant gives homeowners no self-help step for this code at all. Its page says: “This is a case for a Vaillant engineer. Don't try to solve it yourself as opening the boiler casing can be dangerous.”

Key facts

The short version, before the detail.

• Meaning: the control board has no valid Device Specific Number, so it does not know which model of boiler it is fitted to.
• Vaillant's exact words: “The Device Specific Number (DSN) is incorrect, missing or does not match the coding resistor”, from its consumer fault-code page.
• It is on Vaillant's own list, written as F.70 with a decimal point, but the page offers no self-help step and sends you straight to an engineer.
• DIY-fixable? No. The only safe actions are noting the pressure, looking for obvious outside damage, and a single reset.
• Engineer job from the start: the number is entered at diagnostic point d.93 in the installer menu, behind the casing.
• Models: we read the F70 wording ourselves in the ecoTEC plus 424 and ecoFIT pure manuals. Vaillant's public list does not tie F70 to any one model, so treat it as applying across the current ranges that share the same board, including ecoTEC pro, ecoTEC exclusive and ecoFIT sustain.
• The biggest clue: F70 nearly always follows a recent repair, so your first call is the firm that fitted the part, not a new engineer. If nobody has worked on your boiler, it usually points at the coding resistor or its wiring instead, and that is still engineer work.
• Smart Plan is a service plan, not insurance, and cover is for future breakdowns. An F70 showing on your display today counts as a pre-existing fault and would not be covered, so book a one-off repair for that.
• Boiler and central heating cover: parts and labour up to £500 a year if the boiler is under 7 years old, or up to £200 a year if it is 7 years or older, and you pay anything above the limit.
• A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, for no-access visits and for early annual-service requests.
• Using a module starts a 12-month agreement period from the first time you use a service. The 14-day cooling-off period ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.
• Hours differ by job: one-off repair visits run Monday to Friday, 09:00 to 17:00, and carry no workmanship guarantee. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00 unless it is a genuine emergency breakdown.

Why has F70 appeared on my boiler?

Almost always because something inside the boiler was recently changed. A new control board arrives with no model identity, so an engineer has to give it the right Device Specific Number for your appliance. Miss that step, enter the wrong number, or replace the display and the board together so no stored copy survives, and F70 is the result.

The other route to F70 is the coding resistor itself. That is the component inside the boiler that the board checks its number against. If it is missing, damaged, the wrong output range, or the cable harness back to the board has broken, the cross-check fails in exactly the same way. This is the route to look at if nobody has worked on your boiler.

Work down the rows below and see which one matches what actually happened to your boiler. That is usually enough to tell you who should be putting it right.

Which F70 cause matches what you are seeing?

Can I fix a Vaillant F70 myself?

No, and it is worth being straight about why. The number the boiler is missing is set in the installer menu behind the casing, and that casing must stay closed. Nothing you can reach will clear F70.

The steps below are still worth doing. They save the engineer a visit's worth of questions, and they help you work out who should be paying for the return trip.

1. Read the code off the display carefully and note whether it shows a decimal point. Vaillant writes its codes that way, so F.70 and F70 are the same code.

2. Look at the pressure gauge and write the reading down. It is not the cause of F70, but the engineer will ask, and it rules other faults in or out.

3. Check whether your other gas appliances still work, and whether the boiler has power at all. That tells the engineer this is a control fault rather than a supply failure.

4. Look over the outside of the boiler and the flue for obvious damage or water. Do not open, unscrew or remove the casing for any reason.

5. Reset the boiler once, following the reset instructions in your Vaillant user guide. Reset only once. If F70 comes straight back, stop and call an engineer.

6. If a heating engineer worked on your boiler recently, call that firm first. F70 nearly always means a part they fitted was never configured, which makes it a return visit on their own work rather than a new job.

7. If nobody has worked on your boiler recently, check whether it is still inside its Vaillant manufacturer warranty. Your installation paperwork or your Vaillant registration will tell you. If a warranty is still running, contact Vaillant first, because the warranty may cover the visit.

Stop there. Entering or correcting the Device Specific Number, restoring the board to factory settings, testing or replacing the coding resistor, checking the harness back to the board, and replacing the board or display are all engineer work, and so is any removal of the casing.

One thing worth planning for. F70 locks the boiler out, so you have no heating and no hot water until an engineer clears it, and appointments run on weekdays, which means a Friday-evening fault can leave you waiting into the following week. Keep warm with heaters designed for indoor use. Never use a gas hob, a gas oven or an unflued portable heater to warm a room, because burning fuel indoors without a proper flue can produce carbon monoxide. If you ever suspect carbon monoxide, call 0800 111 999.

Vaillant's own instruction for this code is blunt: “This is a case for a Vaillant engineer. Don't try to solve it yourself as opening the boiler casing can be dangerous.”

Is it F70, or one of the codes next to it?

F70 sits in a run of codes that are easy to mix up, and none of its neighbours mean the same thing. F.68 is an unstable flame signal, with causes including air in the gas, gas flow pressure too low, a blocked condensate duct or flue gas recirculation. F.71 is a flow NTC fault, where the flow temperature sensor reports a constant value, is wrongly positioned on the pipe, or has failed. F.72 is a flow and return sensor fault, where the temperature difference between the two is too great.

F.73 and F.74 are both water pressure sensor signal faults: on F.73 the sensor signal sits below the correct range, on F.74 it sits above it. Neither means your water pressure itself is wrong. F.75 means the pressure change is too low when the pump starts, and it is the one code near F70 where Vaillant tells homeowners they may be able to top the pressure back up themselves. F.62 is a flame switch-off detected after a delay, pointing at the gas valve, the circuit board or the ignition electrode.

Two things worth knowing about the code itself. Vaillant writes it with a decimal point, so F.70 and F70 are the same code, and “F.7” is not a Vaillant code at all: it turns up in searches where the last digit was dropped or the display was misread. F.69 does not appear in the current ecoTEC and ecoFIT fault tables we read, which jump straight from F.68 to F.70, so treat any F69 meaning you find online with suspicion.

When should I call a Gas Safe registered engineer?

Straight away, once you have reset once and the code has come back. There is no waiting-and-seeing with F70, because nothing about it improves on its own.

Call the firm that last worked on the boiler before you book anyone new. If they fitted the board or the display, the missing Device Specific Number is unfinished work on their repair, and putting it right needs the correct number for your model. Under the Consumer Rights Act 2015, a service must be carried out with reasonable care and skill, so it is fair to ask them to come back and finish the job. Whether they charge for the return visit is a conversation to have with them, and not something we can promise on their behalf.

If nobody has worked on your boiler, check the warranty before you pay anyone. Your installation paperwork or your Vaillant registration will say whether a manufacturer warranty is still running, and if it is, contact Vaillant first, because the warranty may cover the visit.

If the boiler is out of warranty, or the original firm will not come back, book a Gas Safe registered engineer to test the coding resistor, its harness and the board. You can book a one-off repair with us, or call us on 0333 772 6247. A one-off repair is a single paid job: the engineer diagnoses the fault and tells you the cost before any work goes ahead. One-off repair visits run Monday to Friday, 09:00 to 17:00, and they do not carry a workmanship guarantee.

A Smart Plan cover plan is a separate thing, and it is worth being clear about what it will and will not do for you tonight. Cover is for future breakdowns, not a fault your boiler is already showing, so an F70 on the display today counts as a pre-existing fault and would not be covered. Book the one-off repair for that. A boiler and central heating module then covers the next fault, with parts and labour up to £500 a year if the boiler is under 7 years old, or up to £200 a year if it is 7 years or older, so anything above that limit you would pay yourself.

A few honest details before you set anything up. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, for no-access visits and for early annual-service requests. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00 unless it is a genuine emergency breakdown, so a fault reported on a Friday evening can wait until the next working day. Within those hours we aim to get an engineer out to you within two working days. Using a module starts a 12-month agreement period from the first time you use a service, the 14-day cooling-off period ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.

Smart Plan is a service plan, not insurance. It arranges and funds covered repairs up to the limits on your plan rather than paying out a cash sum, and you are covered only for the modules you have chosen.

One safety note. F70 is a control-board fault, and the boiler has locked itself out rather than fire at an output it cannot verify. But if you ever smell gas or suspect carbon monoxide, do not wait for an engineer: call the National Gas Emergency Service on 0800 111 999 straight away.

Vaillant F70 fault code FAQs

No recent repair, or the original firm won't come back?

Book a one-off repair and we will arrange a Gas Safe registered engineer to come out and test the coding resistor, its harness and the board. A one-off repair is a single paid job, with the cost confirmed before any work goes ahead. One-off repair visits run Monday to Friday, 09:00 to 17:00, and they do not carry a workmanship guarantee. Before you consider anything else, one important point: a cover plan is for future breakdowns, not the F70 your boiler is showing now, which counts as a pre-existing fault and would not be covered. A Smart Plan boiler and central heating module covers the next fault instead, with parts and labour up to £500 a year if your boiler is under 7 years old, or up to £200 a year if it is 7 years or older. It is a service plan, not insurance, so it funds covered repairs rather than paying out a cash sum. Before you sign up: a £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, no-access visits and early annual-service requests. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00 unless it is a genuine emergency breakdown, and within those hours we aim to get an engineer out to you within two working days. Using a module starts a 12-month agreement period once you first use a service, and leaving early after that costs the remaining months or 75% of the outstanding balance. Smart Plan comes from UK Boiler Company Ltd, trading since 2014. Call us on 0333 772 6247.

Helpful links

• Build your boiler cover
• Vaillant boiler help and guides
• All Vaillant fault codes
• Vaillant F71 fault code
• Vaillant F72 fault code
• Vaillant F62 fault code
• Vaillant fault-code help hub
• Boiler repair
• Boiler & central heating cover
• Vaillant official fault-code list
• Check an engineer on the Gas Safe Register