What F26 means
The gas valve assembly controls how much gas reaches the burner. On the ecoTEC models that use F26, a small stepper motor on the valve adjusts the gas flow so the boiler can turn its output up and down. If the board cannot drive that motor, or the motor does not respond, the boiler cannot set its gas rate safely, so F26 locks the boiler out rather than showing a warning.
Vaillant's ecoTEC plus installation and maintenance instructions (document 0020244997) list F26 as "Fault: Fuel valve not working", with the cause "Gas valve assembly stepper motor defective or not connected". The ecoTEC pro instructions (document 0020244995) give the same fault. "Fuel valve" is simply Vaillant's term for the gas valve.
Which Vaillant boilers use F26
- ecoTEC plus VU and VUW ../5-5 regular, system and combi boilers, including the combi 825, 832, 835 and 838 (document 0020244997).
- ecoTEC pro 24 and 28 combis, VUW 246/5-3 and 286/5-3 (document 0020244995).
F26 does not appear in the 2021 fault table for the ecoTEC plus ../6-5 OVZ open-vent regular boilers (document 0020238195) or in the table for the VU 246/6-5 OVZ LPG model (document 0020239015). If your boiler is one of those and shows F26, check the code in that boiler's own manual before relying on this page.
What causes F26
Vaillant's table gives two causes, and they lead to very different repairs.
The stepper motor is not connected
The motor is wired to the board through a plug and the boiler's cable harness. Vaillant's checklist for engineers covers the stepper motor's plug and cable, the multiple plug and the cable harness. A loose plug or damaged cable can produce F26 with a perfectly good valve. It is worth mentioning if anyone has recently worked inside the boiler, for example during a service or a repair to a nearby part.
The stepper motor or gas valve is defective
If the connections are sound and voltage is reaching the motor, the valve assembly itself is at fault. Vaillant is clear on how to tell: if the fault comes back after being cleared, the gas valve assembly is defective.
F26 compared with other Vaillant gas codes
| Code | What it means | Typical first suspect |
|---|---|---|
| F26 | Gas valve not working (stepper motor) | Valve motor wiring, then the valve |
| F61 | Gas valve control fault | Valve, wiring or board |
| F62 | Gas valve switch-off delay | Valve not closing quickly enough |
| F28 | Ignition failure on start-up | Gas supply, condensate or ignition parts |
The useful distinction is that F28 can be caused by a problem outside the boiler, such as the gas supply. F26 is about the valve's motor inside the boiler, so checking your gas supply will not clear it, though it does help rule out a supply problem if you are unsure what you are seeing.
What you can check
None of these steps go near the gas valve. They rule out simpler explanations and collect the details an engineer will ask for.
- Write down the code exactly, and whether the boiler showed F61, F62 or F28 before it.
- Check whether your other gas appliances, such as the hob, are working, and that a prepayment meter has credit. If they are not working either, the problem is the supply, not F26.
- Press reset as your manual describes. Vaillant's own test for this fault is whether it returns after being cleared, so a reset gives useful information.
- If F26 comes back, stop after two or three attempts in total and leave the boiler off.
- Note the boiler's model and serial number from the front label so the engineer can bring the right valve if one is needed.
Never touch or adjust the gas valve, and never remove the casing. If you smell gas at any point, turn the supply off at the meter if it is safe to do so, open doors and windows, do not use electrical switches and call the National Gas Emergency Service on 0800 111 999.
What the engineer will do
Following Vaillant's checklist, the engineer will check the stepper motor's plug and cable, the voltage reaching it, the multiple plug and the cable harness. Expect them to test the connections first, since a reseated plug or repaired cable is a much smaller job than a new valve. If those are all sound, or if F26 returns after the fault has been cleared, they will replace the gas valve assembly, and you can ask them to note on the job sheet that the combustion checks were done. After any work on the gas valve, the engineer has to check that the boiler burns correctly before leaving it running. Our guide on what to do if you smell gas near a boiler covers the emergency steps if anything feels wrong afterwards.
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Plug, cable harness or wiring repair | Typically £100 to £180 |
| Gas valve replacement | Typically £180 to £350 |
If your ecoTEC is still under guarantee, contact Vaillant or your installer before booking a repair. Other ecoTEC codes are listed on our Vaillant fault codes page. When you book, tell the engineer the code, whether a reset cleared it even briefly and how long the boiler ran before F26 returned, as that points them at the wiring or the valve before they open the boiler.