Vaillant F.26 fault codewhat it means and how to fix it
Vaillant call this "fuel valve not working": the stepper motor that drives the gas valve is not responding, so the boiler cannot open the gas and will not light.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.26
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.26 means
The gas valve on a current ecoTEC is not a simple open-or-shut solenoid. Its modulation is driven by a stepper motor that the board positions to match the demand, and F.26 is the board reporting that the motor is "defective or not connected".
The ecoTEC plus and pro tables give the engineer the same check list — the stepper motor's plug, cable, "passage for rinsing" and voltage, then the multiple plug on the board and the cable harness — and the ecoTEC exclusive expands the causes to the motor not connected, the multiple plug on the PCB not seated, an interruption in the harness, the motor itself and the electronics. The order matters: three of the five are wiring, which an engineer proves with a meter before ordering a gas valve, and the phrase "passage for rinsing" refers to the small vent that lets the motor's diaphragm breathe, which can block.
F.26 belongs to the ecoTEC plus ..6/5-5, the pro and the exclusive. The ecoFIT, ecoTEC sustain and 2022 CS/1-5 tables do not carry it, and neither do the 2005 to 2012 ecoTEC or the legacy ranges, so a boiler with a different display showing 26 is worth a second look at the code. There is nothing on Vaillant's list a householder can act on; every item is behind the sealed case or inside the gas train.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Stepper motor plug or cable disconnected | Vaillant's first check. The motor connector sits on the gas valve where it can be disturbed during a service, and the board reads an open motor circuit as a valve that will not move. | Common |
| Break in the cable harness or multiple plug | The exclusive table names the multiple plug on the PCB and an interruption in the harness. Both are proved before any part is changed. | Common |
| Stepper motor failed | The motor has stopped responding to the board. It is replaced with the gas valve assembly, the expensive outcome on this list. | Occasional |
| Electronics fault | Vaillant's last cause. The board's driver stage for the motor has failed and a good valve on a sound harness still reads F.26. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£380
Time on site
1 hour
A connector or harness repair is a call-out of £90 to £150. A gas valve assembly is £280 to £380 fitted; a board is £250 to £400.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant ecoTEC gas valve
0020053968£110–£210Authorised Vaillant parts distributor
Price checked 28 September 2026
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.62Stop — call an engineerThe gas valve is not shutting off when the boiler tells it to. This is a gas safety fault and the boiler must not be used until an engineer has seen it.Related: gas valve and supply
- F.61Get it checkedThe gas valve control circuit has faulted. The boiler will not fire and this is firmly an engineer's job.Related: gas valve and supply
- F.53Get it checkedThe gas is not flowing through the valve the way the boiler expects. Vaillant's list runs from low gas pressure to a wet or blocked filter under the venturi cap.Related: gas valve and supply
- F.54Get it checkedA gas supply fault, usually shown alongside F.28 or F.29. It means gas is reaching the appliance at too low a pressure.Related: gas valve and supply
- F.42Get it checkedA fault with the coding resistor, the small component that tells the board which gas the boiler is set up for. The boiler will not run until it knows.Related: gas valve and supply
- F.88Get it checkedThe board cannot see the gas valve. Vaillant print F.88 as the valve not connected or connected incorrectly, or a harness fault, and the boiler will not run until the circuit is proved.Related: gas valve and supply
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.26
Is F.26 a gas leak?
No. The valve is being told to move and is not answering, so no gas is being admitted. The boiler has locked out. If you smell gas at any time, that is a separate matter: turn off at the meter and call the National Gas Emergency Service on 0800 111 999.
Can I reset it?
Once, using the fault clearance key on your boiler. If the motor connector is intermittent the boiler may run for a while; if the code is straight back, the circuit is open and only an engineer can trace it.
Will I need a new gas valve?
Not necessarily. Vaillant list the plug, the cable and the harness before the motor, and a Gas Safe engineer will measure those first. A valve is the outcome when the motor itself has failed.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.