Vaillant F.67 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.67 fault code mean?
The F.67 code appears on Vaillant boilers when the control board receives a flame signal that falls outside the expected range — in other words, the boiler's flame detection circuitry is reporting something that doesn't add up. The boiler can't confirm a stable, genuine flame during ignition, so it shuts itself down as a precaution. In practice this means no heating and no hot water. You may notice the boiler attempting to fire two or three times before locking out and displaying F.67. Although there is no immediate safety emergency, the fault does need proper investigation because it often points to a failing electronic component, moisture ingress, or a wiring problem rather than anything the boiler can recover from on its own.
General guidance only — not a substitute for professional advice. 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
- Faulty or contaminated flame sensor (ionisation electrode) Common
The flame sensor — sometimes called the ionisation electrode — measures a tiny electrical current through the burner flame to confirm it is present. Over time the electrode tip can become coated in combustion deposits, corrode, or simply fail. When it can't produce a clean signal the PCB sees a reading it doesn't trust and throws F.67. This is the single most frequent cause of this code, particularly on boilers that haven't been serviced recently.
- Moisture ingress affecting electronics Common
Condensate, a roof leak, or dampness during installation can introduce moisture onto the PCB or the wiring loom. Even a small amount of moisture can distort the low-voltage flame signal to the point where the boiler considers it implausible. Vaillant boilers manufactured before approximately 2011 are especially susceptible because of the way their flame detection circuitry was designed.
- Damaged, loose, or corroded wiring and connections Common
The leads connecting the flame sensor to the PCB carry a very small signal. Any break, loose terminal, or corrosion along that path can cause the signal to drop out or fluctuate, mimicking a faulty flame. Vibration over years of operation can work connectors loose, and this is often found alongside other symptoms such as intermittent lockouts.
- PCB or ignition module fault Sometimes
If the circuit board itself — or the ignition module it drives — has been damaged by a power surge, heat stress, or age, it may generate or misread the flame signal incorrectly. PCB faults are harder to diagnose without specialist test equipment but should be considered once the sensor and wiring have been checked.
- Unstable or interrupted gas supply Sometimes
If the gas pressure at the boiler drops below what the burner needs to sustain a flame, the flame can extinguish partway through the ignition sequence. The sensor then reports a loss of flame, which can register as an implausible signal. Checking that other gas appliances in the property are working normally is a quick first step.
How to fix it
- Check your other gas appliances are working DIY safe
Turn on a gas hob or check another gas appliance in the property. If nothing is working, the issue is with your gas supply rather than the boiler itself — contact your gas supplier. If other appliances work normally, the fault is internal to the boiler and you can move on.
- Reset the boiler once or twice DIY safe
On most Vaillant ecoTEC models the reset button is marked with a flame icon crossed by a line, located on the main display panel. Press and hold it for around three seconds until the display changes. Allow the boiler a full minute to attempt a restart. If the fault clears and the boiler runs normally, monitor it over the next day or two. If F.67 returns, do not keep resetting — repeated unsuccessful resets will not fix an underlying component fault and can mask useful diagnostic information for an engineer.
- Check for obvious signs of moisture around the boiler casing DIY safe
Look for watermarks, rust streaks, or condensation on or around the boiler casing and the flue connection. If you can see evidence of a leak or damp, note where it appears to be coming from and mention it to the engineer — moisture ingress is a significant clue for diagnosing F.67.
- Have a Gas Safe engineer inspect and test the flame sensor and its wiring Gas Safe engineer
An engineer will remove the burner assembly to access the ionisation electrode, clean it or replace it if worn, and check the signal lead and connector for corrosion or damage. This single check resolves a large proportion of F.67 faults and is typically the first thing an experienced Vaillant engineer will do.
- Have the engineer inspect the PCB and ignition module for moisture or surge damage Gas Safe engineer
If the flame sensor and wiring are in good condition, attention moves to the control board and ignition module. The engineer will look for discolouration, corrosion, or damaged components and may test the PCB outputs with a multimeter. A faulty PCB or ignition module will need to be replaced like-for-like.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
Beyond a basic reset and visual check, all work on the ignition system, flame sensor, wiring, and PCB must be carried out by a Gas Safe registered engineer. It is both unsafe and illegal for an unregistered person to carry out gas work in the UK. When booking, mention the F.67 code and any history of power surges or moisture you've noticed — this will help the engineer bring the most likely replacement parts.
Parts you may need
- Ionisation / flame sensor electrode · from £25
- Ignition lead and HT electrode kit · from £35
- PCB (printed circuit board) · from £180
- Ignition module · from £95
- Wiring loom / connector harness · from £45
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 £120–£320, depending on the underlying cause.
Frequently asked questions
Is the Vaillant F.67 fault dangerous?
No, F.67 is not an immediate safety emergency. The boiler has shut itself down precisely because it cannot confirm a proper flame, which is the safe thing to do. However, you should not repeatedly override the lockout by resetting the boiler without investigating the cause. Get it looked at promptly so your heating and hot water are restored and the underlying fault — whether a sensor, wiring issue, or moisture — is properly resolved.
Can I fix the F.67 fault myself?
The one DIY step you can take is a single boiler reset. Beyond that, the fault involves components inside the boiler — the flame sensor, ignition wiring, and potentially the PCB — all of which must only be worked on by a Gas Safe registered engineer. Attempting gas or electrical work inside a boiler without registration is illegal in the UK and voids any remaining manufacturer warranty.
How much does it typically cost to fix a Vaillant F.67 fault?
Most people pay somewhere between £120 and £320 all in. A straightforward fix — cleaning or replacing the flame sensor and checking the wiring — usually sits in the £120–£200 range including labour. If the ignition module needs replacing, expect £200–£320. A PCB replacement is the costliest common repair and can push the bill to £350–£450 or more once labour is included, though this is less frequently the root cause. Always get a written quote before work begins.
Why is the F.67 fault more common on older Vaillant boilers?
Vaillant boilers manufactured before around 2011 used a flame detection circuit design that is more sensitive to moisture and voltage fluctuations than later versions. Over time, seals can degrade, allowing condensation to reach electronic components more easily, and the components themselves age and lose tolerance. If your boiler is over 12–15 years old and showing F.67 repeatedly, it may be worth asking an engineer whether a repair represents good value compared with a replacement, particularly if other components are also showing their age.
Will an annual boiler service help prevent F.67 coming back?
Yes, meaningfully so. During a Vaillant service an engineer will clean the ionisation electrode, inspect the ignition components, check wiring connections, and look for any signs of moisture ingress. Catching a degraded electrode or a loose connector before it triggers a lockout is far cheaper than an emergency call-out. Annual servicing is also a condition of most boiler warranties, so it is worth keeping up to date regardless.