The Vaillant F04 fault code means the boiler failed to ignite / flame not detected. To protect you and the appliance, the boiler locks out and stops providing heating and hot water until the issue is resolved. Typical triggers include gas supply problems, ignition electrode issues, flame sensing faults, or flue/air problems. Do not remove the boiler case – internal checks are for Gas Safe engineers only.
| Likely Cause | What’s Happening | Who Should Act |
|---|---|---|
| Gas supply issue or low inlet pressure | Insufficient gas reaches the burner, so the boiler can’t light | Homeowner can check meter credit/valves; engineer tests pressure & supply |
| Ignition electrode / lead fault | Weak or no spark prevents ignition | Gas Safe engineer to test/replace electrode or lead |
| Flame sensor (ionisation) fault | Boiler lights but can’t detect a stable flame, so it shuts down | Engineer to clean/test/replace sensor; verify earth & polarity |
| Blocked or restricted flue/air intake | Poor combustion air or blocked exhaust prevents safe lighting | Homeowner can check external terminal is clear; engineer inspects flue |
| PCB or wiring connection fault | Ignition sequence isn’t controlled/read correctly | Engineer to test wiring, connectors and PCB |
Why this matters: Multiple reputable UK sources describe F.4 / F04 as an ignition or flame-detection failure leading to lockout. Always use a qualified engineer for internal diagnostics and repair.
Check gas: Is your gas meter in credit? Are other gas appliances working? Is the emergency control valve fully open?
Check water pressure: Most Vaillant boilers need ~1.0–1.5 bar cold. Top up via the filling loop if your manual allows.
Check outside flue terminal: Make sure it’s not blocked by debris, ice, or foliage.
One reset only: After the checks above, try one reset from the user controls. If F04 returns, stop and call an engineer.
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| Engineer Step | Detail | Outcome |
|---|---|---|
| Inlet gas pressure & tightness | Measure standing/working pressure; verify supply, valves & meter | Confirms or rules out supply issue |
| Ignition components | Inspect/clean/replace ignition electrode & lead; check spark gap | Restores reliable spark |
| Flame detection | Test ionisation current; clean/replace flame sensor; check earth/polarity | Ensures stable flame recognition |
| Combustion air & flue | Check fan operation, condensate, flue integrity/obstructions | Restores safe combustion conditions |
| Electronics & wiring | Continuity tests, connector inspection, PCB diagnostics | Repairs control or communication faults |
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| Repair Item | Indicative Cost (UK) |
|---|---|
| Diagnostic visit (Gas Safe) | £70 – £120 |
| Ignition electrode & lead replacement | £110 – £190 |
| Flame sensor (ionisation) replacement | £100 – £180 |
| Flue/air fault rectification (minor) | £90 – £180 |
| PCB replacement (if required) | £250 – £450 |
Prices vary by model, parts, region and access. We’ll confirm costs upfront.
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Annual boiler service (protects efficiency and validates manufacturer warranty)
Keep the flue terminal clear year-round
Consider a home care plan to spread maintenance cost
If your boiler is older and frequently faults, a new boiler installation may be more economical long-term
For more information on other Vaillant fault codes, you can refer to the following:
Consider exploring the following services to maintain and enhance your heating system:
For more information on Vaillant Boilers you can refer to the following articles:
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