Error F.26

Safety precautions:

1) If you smell gas, evacuate the property immediately, call the gas emergency number and do not operate electrical switches or the boiler.

2) Always isolate electrical supply to the boiler and turn off the gas at the meter before any internal inspection. If you are not competent, do not isolate gas/electrics yourself — call a Gas Safe engineer.

3) Do not open the boiler casing, do not attempt to dismantle the gas valve or stepper motor, and do not attempt any ignition or gas-flow tests yourself.

Initial checks a homeowner can do (safe, no internal access):

1) Note the displayed fault code and any accompanying LED pattern or history. Photograph the display for the engineer.

2) Try a simple reset: switch the boiler off at the programmer or the mains isolator, wait 1 minute, switch back on and observe whether F.26 returns. Record whether the fault is immediate or appears after attempting to fire.

3) Check household gas supply: verify other gas appliances (hob, oven) work to confirm the property gas supply is present. If other appliances also have no gas, contact Gas Distribution emergency number.

4) Check boiler water pressure on the pressure gauge. If pressure is very low (below about 1.0 bar) repressurise to the recommended operating pressure (usually ~1.0–1.5 bar; check your boiler handbook) and try a reset. Low pressure can cause other fault cascades.

5) Look externally at the boiler for visible leaks, water ingress, or obvious corrosion around the valve area and wiring. Take photos if you see damage; do not touch or remove covers.

6) Ensure any external controls (room thermostat, programmer, TRVs) are calling for heat when you test so the engineer can reproduce the condition.

Specific diagnostic steps for an engineer (what will happen once a professional attends):

1) Confirm identity and safety certificates (Gas Safe) and make the boiler safe to work on. Check for gas leaks and electrical isolation.

2) Read full fault memory and live diagnostic values from the PCB. Note any related faults (F28/F29/F54 etc.) which can point to gas valve issues or PCB problems.

3) Visual inspection of wiring harness and connectors, especially the X25 connector and the yellow connector to the gas stepper motor. Check for loose pins, burnt contacts, water damage, or corrosion.

4) Verify continuity and proper signals between the PCB and the stepper motor/gas valve. Measure voltages/drive signals while attempting to operate the valve. Check stepper motor resistance/impedance values against manufacturer data.

5) Test the stepper motor operation: observe if it attempts to move, whether it stalls, or shows intermittent operation. Inspect for mechanical seizure or foreign debris.

6) Check related sensors and components that can cause valve lockouts: flow/return NTC thermistors, pump operation and speed, airlocks, system blockages, and any evidence of condensation or water damage on the PCB.

7) If electrical connections and signals are correct but the valve/stepper motor does not operate, replace the gas valve assembly with the correct Vaillant part for that model. The engineer will ensure the correct part number is used and will follow manufacturer procedures to fit and commission the valve.

8) If the valve and stepper are functional but faults persist, test and, if necessary, replace the PCB or repair wiring harnesses. After any replacement, the engineer will run gas-pressure and combustion checks, re-calibrate where required, and carry out a full safety and leak test.

When to call a professional and final notes:

1) Call a Gas Safe registered heating engineer as soon as the fault persists after a reset or if you see signs of water damage, burning on wiring, or repeated faults. Do not attempt internal repairs yourself.

2) Provide the engineer with the boiler model, serial number, the exact fault code (F.26), any other fault codes observed, and the results of your simple checks (pressure reading, whether reset cleared the code temporarily, photos of visible damage).

3) After repair, the engineer will reset the boiler and test it under normal running conditions. Keep service records and any replaced part numbers for future reference.

Remember: F.26 is a gas-valve related lockout. It must be diagnosed and repaired by a qualified Gas Safe engineer. Homeowner checks should be limited to safe, external steps and gathering information to help the engineer diagnose the fault promptly.