Vaillant F71 fault code: flow sensor fault explained

What does the Vaillant F71 fault code mean?

The Vaillant F71 fault code means the flow NTC, the sensor that reads the temperature of the water leaving the boiler, is stuck, reporting the same value even while the burner is firing. Common causes are the sensor coming loose from the flow pipe, a defective thermistor, or damaged or short-circuited wiring between the sensor and the circuit board. You can check the boiler's displayed water pressure and look for obvious damage without opening the casing; testing or replacing the sensor and its wiring is an engineer's job.

F71 doesn't appear on Vaillant's own consumer fault-code page, which lists F70 and F72 either side of it but skips F71 entirely. The only official definition comes from Vaillant's installer and servicing manual, which describes it as “Flow NTC reports constant value (stuck at)”, caused by a defective Flow NTC. In plain terms, the boiler can't trust a sensor reading that never changes, so it locks out rather than risk running without knowing the real flow temperature.

Key facts

The quick version, before the detail.

• Meaning: the flow NTC sensor reports a stuck, unchanging reading (Vaillant's installer-manual wording).
• DIY-fixable? No, the only safe steps are a pressure and visual check plus a single reset.
• Engineer job if: the fault returns, since testing or replacing the sensor means opening the casing.
• Most common on: the Vaillant ecoTEC range.
• Not on Vaillant's own fault-code page: F71 is documented only in the installer and servicing manual, unlike F70 and F72 either side of it.
• Smart Plan 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 over 7.
• A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, if the engineer cannot get access, and for early annual-service requests.
• Cover-plan call-outs run Monday to Friday, 08:00 to 18:00. Outside those hours you are only seen sooner for a genuine emergency breakdown, so a Friday-evening or weekend fault can wait until Monday.
• Using a module starts a 12-month agreement period the first time you use a service. There is a 14-day cooling-off period, but it ends once a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.

Why does the F71 code appear?

F71 points at the boiler's flow NTC rather than at the gas side or the flame. The common causes are the sensor coming loose or disconnecting from the flow pipe, the thermistor itself failing, or damaged or short-circuited wiring between the sensor and the circuit board. Less commonly, the sensor may be sitting in the wrong position on the supply pipe. It shows up most on the Vaillant ecoTEC range.

The table below runs through the likely causes, what you'll notice, and who can safely deal with each one.

F71 causes and who fixes them

Can I fix a Vaillant F71 myself?

There's no safe way to reach the flow sensor or its wiring yourself, they sit inside the casing, so the checks you can make are limited. Work through them in this order.

1. Check the water pressure gauge on the boiler is showing a normal reading.

2. Look around the outside of the boiler for any obvious leaks or damage, without opening the casing.

3. Reset the boiler once, following the steps in your Vaillant manual. Reset only once.

Stop there. Testing or replacing the flow NTC, checking the wiring at the circuit board, or repositioning the sensor all mean opening the casing, which should be left to an engineer. If F71 comes back after a reset, book an engineer rather than resetting again.

F71 vs F70 and F72: what's the difference

F71, F70 and F72 sit next to each other on Vaillant's fault-code list, but they report different problems. F70 is a device-specific-number or coding-resistor fault. F72 means the spread between the flow and return temperatures is wrong. F71 is different again: it means the flow sensor's reading is frozen, reporting the same value even while the burner is lit.

One thing sets F71 apart from its neighbours. Vaillant's own consumer-facing fault-code page lists F70 and F72, but skips F71 entirely. The only place F71 is documented is Vaillant's installer and servicing manual. If you've looked up your code on Vaillant's website and come away empty-handed, that's why, not because the fault doesn't exist.

When to call a Gas Safe registered engineer

If you've checked the pressure, looked for obvious damage and reset once, but F71 keeps coming back, it needs a Gas Safe registered engineer to test the flow sensor and its wiring, or replace it. That's work inside the boiler casing, not a DIY job.

A repair that needs a new flow sensor or a wiring fix is a bigger job than a simple reset, so the parts and labour can add up. That's worth weighing against how you'd rather cover it, a one-off repair or an ongoing module.

You can book a one-off repair with us and a Gas Safe registered engineer will come and fix it, or call us on 0333 772 6247. A Smart Plan boiler and central heating module covers parts and labour up to £500 a year if your boiler is under 7 years old, or up to £200 a year if it's over 7. If the repair costs more than your cover limit, you would pay anything above the limit yourself. Cover is modular, so you only pick what you want.

A few things worth knowing before you set up cover. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan doesn't cover, if the engineer can't get access, and for early annual-service requests. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00; outside those hours you're only seen sooner for a genuine emergency breakdown as set out in your terms, so a fault on a Friday evening or over the weekend can wait until Monday. And using any module starts a 12-month agreement period from the first time you use a service, there's a 14-day cooling-off period, but it 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. In plain terms, it's a membership that arranges and funds covered repairs up to the limits on your plan, rather than a regulated financial product, it doesn't pay out a cash sum, and cover is limited to the modules you've chosen and the limits set out in your terms. It's provided by UK Boiler Company Ltd, which has traded since 2014 and looked after over 15,000 customers.

One safety note: F71 is a sensor fault, not a gas one, but if you ever smell gas or suspect carbon monoxide, don't wait for an engineer, call the National Gas Emergency Service on 0800 111 999 straight away.

Vaillant F71 fault code FAQs

Seeing F71? We'll come and fix it for you.

Book a one-off repair and a Gas Safe registered engineer will test the flow sensor and its wiring, or set up a Smart Plan boiler and central heating module so covered repairs are handled up to your cover limit, up to £500 a year for a boiler under 7 years old, or up to £200 a year if it's older. It's a service plan, not insurance, so it funds covered repairs rather than paying out a cash sum. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan doesn't cover, no-access visits and early annual-service requests. Cover-plan call-outs are Monday to Friday, 08:00 to 18:00 unless it's a genuine emergency. Using a module starts a 12-month agreement period once you first use a service; leaving early costs the remaining months or 75% of the outstanding balance.

Helpful links

• All Vaillant fault codes
• Vaillant F72 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