Safety switch off flue gas temperature too high
The F25 error code on Vaillant boilers indicates safety switch off flue gas temperature too high.
Vaillant F25 means: safety switch off flue gas temperature too high. The usual causes are break in plug connection for optional flue gas safety thermostat (stb), or break in cable harness. A Gas Safe registered engineer should diagnose the root cause.
What Does F25 Mean
The F25 code is shown by the boiler’s control board when it detects safety switch off flue gas temperature too high. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows F25 and the boiler stops until the fault is cleared |
Common Causes
Causes documented in 10 technical sources.
| Cause |
|---|
| Break in plug connection for optional flue gas safety thermostat (STB), |
| break in cable harness |
Safety Warning
F25 is a gas-related fault. Never attempt to repair or adjust the gas valve, burner assembly, or flue system yourself. All gas work must be carried out by a Gas Safe registered engineer (UK) or licensed HVAC technician (US). Repeated resets after ignition failure can cause unburnt gas to accumulate inside the combustion chamber — this is a serious safety hazard. If you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property, and call your gas emergency service immediately. Gas-related fault. Do not open the boiler casing or attempt repairs to the gas valve, burner, ignition or flue. Repeated resets are unsafe. Diagnosis and repair must be carried out by a Gas Safe registered engineer.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
Step-by-Step Diagnosis
1. Check the circulation pump — verify it is running and water is flowing (feel the pipes either side of the pump for temperature difference). 2. Check system valves — ensure all isolation valves and zone valves are fully open. 3. Check the system for air locks — bleed radiators and check for gurgling noises. 4. Inspect the heat exchanger — check for limescale build-up or sludge. 5. Test the flow and return thermistors — compare the temperature reading with an external thermometer. 6. Check pump speed setting — increase if necessary. 7. If the system has a filter (magnetic or inline), check if it needs cleaning
How to Fix It
Bleed the system of air
flush the heating system if sludge is present (consider a power flush)
replace the pump if it is not circulating
descale or replace the heat exchanger if blocked with limescale
open any closed valves
replace faulty thermistors
Refer to the Vaillant service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
Diagnostic Tools
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Affected Models
Vaillant models that can display fault code F25:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Vaillant models | — | Possible |
Frequently Asked Questions
Bleed the system of air. According to the technical documentation, this is the first check on the list for F25.
F25 is classified as a high-severity fault: it involves gas, combustion or flue components. Do not keep resetting the boiler and do not attempt the repair yourself — call a Gas Safe registered engineer.