If your Vaillant ecoTEC Pro, ecoTEC Plus, ecoFIT Pure or ecoMAX is locked out on F26 (red flashing light, no heating, no hot water), the boiler has detected a serious problem with the electronic gas valve.
F26 means “Gas valve stepper motor feedback fault”.
The modern gas valve uses a tiny electric stepper motor to precisely control the gas/air mixture. The boiler sends a command to the motor, but the feedback signal says the valve didn’t move to the correct position – so it shuts down instantly for safety.
This is one of the more expensive Vaillant faults and is increasingly common on boilers from 2012–2019.
What Actually Causes F26 on Vaillant Boilers
Faulty Stepper Motor Inside the Gas Valve
The motor itself burns out or the gears strip – the single most common cause.
Broken or Corroded Wiring Harness to the Gas Valve
The little 6-pin plug on top of the valve suffers from years of heat cycling and the pins go green or the clip snaps.
Water Damage to the Low-Voltage Side of the PCB
Condensate drips from the sump onto the gas-valve driver circuit (very common on ecoTEC Plus 2012–2016).
Gas Valve Driver Circuit Blown on the Main PCB
A short-circuit from water or a surge fries the driver chip.
Extremely Rare – Actual Blockage in the Gas Valve
Only if the boiler has been run without a filter in a very dirty system.
Safe DIY Checks (Almost None)
Reset the boiler – hold the spanner/reset button for 5 seconds.
Look underneath for any dripping condensate – a visible leak is often the root cause.
That’s literally it. You cannot test or repair a stepper gas valve yourself.
When You Must Call a Gas Safe Engineer (Every Single Time)
F26 is 100 % a gas-appliance fault. Only a registered engineer can:
- Test the stepper motor and harness
- Replace the gas valve (£300–£400 part)
- Replace the wiring harness
- Replace the PCB if the driver is blown
- Carry out full combustion calibration after repair
Check they’re Gas Safe at www.gassaferegister.co.uk and ideally Vaillant-trained.
Typical 2025 Repair Costs for Vaillant F26
- New wiring harness only (rarely fixes it): £130–£190
- New gas valve (most common fix): £420–£580 fitted & commissioned
- New main PCB (if driver damaged): £480–£680 fitted
- Gas valve + PCB (worst case): £850–£1,100
- Fixed-price “F26 stepper fault” packages: £520–£720 (many Vaillant specialists now offer this)
At £700+ many owners choose to replace the boiler instead.
How to Prevent F26 Happening Again
Annual service by a Vaillant Advance engineer – they spot early sump leaks before water hits the board.
Fit a condensate pump or pro kit in loft installations.
Use a quality magnetic filter and inhibitor from day one.
Frequently Asked Questions About Vaillant F26
What’s the difference between F26, F61 and F62?
F61 = no control signal at all
F62 = valve slow to close
F26 = stepper motor didn’t reach the correct position
All usually end up needing the same parts.
Will resetting clear F26?
Never permanently – it returns within seconds.
Is F26 covered by warranty?
Yes – if still in the 5–10 year period and serviced annually by a Vaillant-approved engineer.
My boiler is only 7 years old – why F26 already?
Condensate dripping onto the PCB is the classic cause on 2012–2016 ecoTEC Plus models – still fully covered under warranty.
Repair or replace?
Under 10 years → repair (£500–£700).
Over 12 years or needs valve + PCB → new boiler usually makes financial sense.
Staring at F26 right now? Drop your exact model and year in the comments and we’ll tell you the most likely repair route and realistic cost in your area.
Stay warm,
The Boilers2Go Team
With years of experience in boilers and home improvements in general, Myles Robinson is a passionate advocate for making home heating simpler, safer, and more affordable for households across the UK. With years of experience in the heating industry, Myles combines hands-on knowledge with a focus on customer-first service, helping homeowners choose the right boiler, understand their systems, and keep their heating running efficiently all year round. His articles break down complex topics into clear, practical advice you can trust.