Meanings, Likely Causes & What To Do Next
If your Vaillant shows an F-code on the display, it’s telling you what’s wrong. Use the quick table below to understand the issue and whether you can try a safe check (like topping up pressure) before calling a Gas Safe engineer.
Safety first: If you smell gas, hear arcing, or the boiler repeatedly locks out—turn it off and call a Gas Safe engineer.
How to use this guide
- Note the exact code on the display (letters + numbers).
- Find it in the relevant group below.
- Try the safe checks.
- If the code returns or you’re unsure, book a qualified engineer.
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Vaillant Boiler Fault Codes
| Code | Plain-English meaning | Quick next step |
|---|---|---|
| F20 | Overheat / temperature limiter activated | Power off, allow to cool; engineer to check circulation/thermostats. |
| F22 | Low water pressure / dry fire protection | Check system pressure is ~1.0–1.5 bar; top up if you know how; persistent drops need an engineer (leak/sensor). |
| F23 | Flow/return temperature difference too high (poor circulation) | Bleed radiators; engineer to check pump/blockages/balancing. |
| F24 | Temperature rise too fast (circulation/pump issue) | As above; likely pump/air/valve problem. |
| F25 | Flue gas temperature too high | Switch off; engineer to inspect flue, sensors, heat exchanger. |
| F27 | False flame detected (flame signal when there shouldn’t be one) | Engineer to test electrodes, PCB, gas valve. |
| F28 | Ignition failed (no flame established) | Check gas supply to other appliances; engineer to diagnose ignition/gas/condensate. |
| F29 | Flame lost during operation (then re-ignition failed) | Often wind/condensate/fuel issues; engineer to investigate. |
| F62 | Gas valve shut-off fault (closing delay) | Engineer only—gas valve/PCB checks. |
| F64/F65 | Electronic sensor/system fault / electronics temperature too high | Power-cycle once; engineer to test NTCs/PCB. |
| F67 | Electronic fault in flame signal | Engineer to check ignition/flame rectification circuit. |
| F70 | Parameter mismatch (PCB/display) | Engineer to re-parameterise/replace board. |
| F72 | Flow/return NTCs out of range (mismatched readings) | Engineer to test/replace sensors/wiring. |
| F75 | No pressure rise detected when pump starts (pump/pressure sensor) | If pressure OK but code persists → engineer; very common on ecoTEC. |
Note: Code wording varies slightly by model (ecoTEC pro/plus, ecoFIT, older Turbomax). Meanings above are consistent across Vaillant guidance and installer references. For exact model phrasing, see Vaillant’s official lookup and your model manual.
The top 5 Vaillant codes (and what they usually mean)
F22 — Low pressure
Most often the system needs repressurising or there’s a small leak. If pressure keeps dropping after topping up, an engineer should check for leaks or a failing pressure sensor.
F28 / F29 — Ignition / flame loss
Can be gas supply interruption, ignition electrodes, blocked/frozen condensate, or flue/air issues. Engineers test gas rate, ignition and condensate run.
F75 — Pump/pressure sensor
The boiler expects a pressure “bump” when the pump starts. If it can’t detect that, it locks out. Usual culprits: stuck/weak pump, sludged or faulty pressure sensor.
F20 / F23 / F24 — Overheat & circulation
These point to poor water movement (air, closed valves, blocked filters, failing pump) or control sensor problems.
F27 — False flame
Flame signal when no gas should be burning—engineer checks electrodes, wiring, PCB and gas valve safety.
When can you try something yourself?
- Check pressure: aim for 1.0–1.5 bar when cool; top up if you’re comfortable doing so.
- Reset once: after a simple issue (like a one-off pressure top-up).
- Condensate in freezing weather: ensure the external pipe isn’t frozen (thaw safely if you know how).
If a code returns repeatedly or relates to gas, ignition, electronics, or sealed components, call a Gas Safe engineer.
Model manuals & official references
- Vaillant official fault-code lookup (by model) and guidance.
- ecoTEC pro installation/maintenance manual (fault message handling, diagnostics menu).
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//CONTACT US
Need help fast?
24/7 emergencies, ~1-hour ETA. Certified plumbing, heating/boiler, drainage, gas and electrical engineers—transparent pricing, fully insured, UK-wide. Call or book online.