Your Vaillant boiler has flashed an error code and you want to know what it means, whether you can fix it yourself, and how much a repair might cost. This article explains the most common Vaillant error codes seen in UK homes in 2026, the likely causes, and the practical steps you can take before calling an engineer.
The single most frequent Vaillant fault is the F.75 error code, which signals a pressure drop or pump blockage, not a gas supply problem. Most other common codes fall into three groups: low pressure (F.22, F.23), ignition failure (F.28, F.29), and fan or flue faults (F.62, F.64, F.65).
The F.75 Error Code Is the Most Common Vaillant Fault and It Usually Means a Pressure Problem
F.75 is the error code Vaillant boilers display most often. It indicates the boiler has detected a pressure drop or a pump blockage, not a gas supply issue (Vaillant UK technical documentation, 2026). The boiler locks out after repeated failed attempts to maintain system pressure above 0.5 bar.
You can attempt a repressurisation via the filling loop before calling an engineer. The filling loop is usually a braided hose or a silver lever beneath the boiler. Turn it slowly until the pressure gauge reads 1.0 to 1.5 bar, then close it fully. If the error returns within 24 hours, a sealed-system leak or failed expansion vessel is likely (Gas Safe Register fault-classification data, 2026).
Do not ignore a recurring F.75. A leaking system wastes water and energy, and a failed expansion vessel can cause pressure to spike dangerously when the boiler heats up.
F.22 and F.23 Indicate Low System Pressure and Repressurise Before Calling an Engineer
F.22 appears when system pressure falls below 0.3 bar. F.23 appears when it drops below 0.1 bar (Vaillant installation and servicing instructions, F.22/F.23 sections, 2026). Both errors prevent the boiler from firing. The display shows the code and the boiler remains off.
Repressurise to 1.0–1.5 bar using the filling loop. Locate the filling loop beneath the boiler, open both valves slowly, watch the pressure gauge rise, then close both valves. If pressure drops again within a week, a plumber should pressure-test the system for leaks (EST boiler-efficiency guidance, 2026).
Common causes of repeated pressure loss include a radiator valve leak, a pin-hole leak in a pipe, or a failed pressure relief valve. An engineer can identify the source with a pressure test.
F.28 and F.29 Signal Ignition Failure and Check Gas Supply Before Resetting
F.28 means the boiler failed to ignite within the safety time window, typically 5 seconds. F.29 means ignition was detected but the flame was lost immediately after (Gas Safe Register technical bulletin TB-026, ignition-fault classification, 2026).
First step: check other gas appliances in your home, such as a gas hob or gas fire, to rule out a supply interruption. If gas is flowing elsewhere, the fault is likely a blocked burner, faulty electrode, or gas valve issue (Vaillant ecoTEC fault-code matrix, 2026).
Do not reset more than three times. Repeated failed ignition can flood the combustion chamber with unburnt gas, creating a safety hazard. If the error returns after two or three resets, call a Gas Safe registered engineer.
F.62, F.64, and F.65 Are Fan or Flue Errors and Do Not Operate the Boiler
F.62 indicates the fan speed is outside the expected range. F.64 means the fan is not running at all. F.65 is a flue-temperature sensor fault (Vaillant technical support manual, fan-error section, 2026).
These codes always require a Gas Safe registered engineer to inspect the fan assembly and flue path. Running the boiler with a fan fault risks incomplete combustion and carbon monoxide production (Gas Safe Register CO-safety guidance, 2026).
The engineer will test fan voltage, clean the fan impeller, or replace the fan module. Do not attempt to reset these errors yourself. If the boiler has displayed F.62, F.64, or F.65, leave it off until an engineer arrives.
F.70 and F.72 Are Sensor Errors and the Boiler May Still Run but at Reduced Efficiency
F.70 means the NTC flow sensor reading is out of range. F.72 means the return sensor reading is out of range (Vaillant spare-parts catalogue, 2026). The boiler may still fire but will limit output to protect itself, leading to longer heating times.
The sensor can be tested with a multimeter by an engineer. Resistance values at 25°C should be approximately 10kΩ. A failed sensor typically costs £20 to £40 for the part plus labour. Replacement takes 30 to 60 minutes (MCS installer fault-diagnosis data, 2026).
If you see F.70 or F.72, you can continue using the boiler but expect slower heating. Book an engineer to replace the sensor when convenient.
Quick Numbers Vaillant Error Codes, Common Causes, and Typical Repair Costs
| Error Code | Common Cause | DIY Fix? | Typical Engineer Cost (parts + labour) |
|---|---|---|---|
| F.22 / F.23 | Low system pressure | Repressurise via filling loop | £80–£150 for leak detection and repair |
| F.28 / F.29 | Ignition failure | Check gas supply; reset once | £120–£250 for burner or electrode replacement |
| F.62 / F.64 | Fan or flue fault | No, do not operate boiler | £150–£300 for fan module replacement |
| F.70 / F.72 | Sensor fault | No, but boiler may run | £80–£150 for sensor replacement |
| F.75 | Pump or pressure problem | Repressurise; if returns, call engineer | £150–£400 for pump or expansion vessel repair |
Costs are based on published repair estimates from Vaillant UK and the Checkatrade 2026 boiler-repair pricing survey (Checkatrade, 2026). Actual costs vary by region and engineer.
How to Reset a Vaillant Boiler After an Error Code and When Not To
Locate the reset button on your Vaillant boiler. It is usually a circular button on the front control panel, sometimes labelled with a flame symbol (Vaillant user manual, reset procedure section, 2026). Press and hold for 3 to 5 seconds until the display shows a normal pressure reading or a flashing flame symbol.
If the error code reappears immediately after reset, do not keep resetting. The underlying fault remains and repeated resets can worsen the problem. Never reset an F.62, F.64, or F.65 error. These require engineer inspection before the boiler can safely operate (Gas Safe Register safety advisory on repeated resets, 2026).
As a general rule, if the boiler has not fired for more than 30 minutes and the error code is still displayed, call an engineer. Do not attempt to bypass the safety lockout.
Which Engineers Can Work on Vaillant Boilers and How to Verify Their Credentials
All gas boiler repairs must be carried out by a Gas Safe registered engineer. This is a legal requirement under the Gas Safety (Installation and Use) Regulations 1998 (legislation.gov.uk). Vaillant does not require MCS certification for boiler repairs. MCS certification is for renewable heating systems such as heat pumps and solar thermal (Vaillant UK service-network FAQ, 2026).
To verify an engineer, ask for their Gas Safe Register ID, then check it at GasSafeRegister.co.uk or call 0800 408 5500 (Gas Safe Register find-an-engineer tool, 2026). Vaillant Advance engineers are factory-trained but are not mandatory. Any Gas Safe registered engineer with boiler experience can work on a Vaillant.
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Frequently Asked Questions
F.75 means the boiler has detected a pressure drop or pump blockage, not a gas supply problem. According to Vaillant UK technical documentation, this is the most common Vaillant error code and locks the boiler out after repeated failed attempts to maintain pressure above 0.5 bar.
Press and hold the reset button on your Vaillant boiler for 3-5 seconds until the error code clears. If the code returns immediately, there is an underlying fault that needs professional attention, as per Gas Safe Register guidance.
Yes, you can repressurise the system to 1.0-1.5 bar using the filling loop. The Energy Saving Trust advises that if the F.22 error returns within 24 hours, you should call a Gas Safe engineer as it indicates a sealed-system leak.
F.28 indicates ignition failure, meaning the boiler has tried and failed to light the gas. Check that your gas supply is on and other gas appliances are working. If the gas supply is fine, a Gas Safe engineer must inspect the ignition components (Vaillant installation instructions, 2026).
Typical Vaillant boiler repairs cost between £120 and £350 depending on the fault, according to Checkatrade data (2026). Fixed-price repairs from Vaillant-approved engineers start around £150, while a full system flush or component replacement can reach £500.
