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Viessmann Boiler Fault Code Guide
Vitodens 050-W, 100-W & 200-W — F2, F4, F9, Eb, 0C and more explained plainly
Your Viessmann boiler has logged a fault code to protect itself from damage — not to be ignored. This guide covers every common Vitodens fault code, what it means, what you can safely try yourself, and when to call a Gas Safe engineer. Thermotec is Wokingham’s Viessmann Accredited specialist.
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installer
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diagnostic
diagnostic
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fix rate
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Boiler showing a fault code?
Fixed price · Upfront quote · Viessmann specialist
Diagnostic fee£96
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ResponseSame day
Fault code index
Viessmann Vitodens fault codes — full reference
Find your fault code below. The severity column shows whether you can attempt a fix yourself (low/info), or whether you need a Gas Safe engineer immediately (high).
| Code | Fault | Common cause | Action | Severity |
|---|---|---|---|---|
| F2 | Burner overheated / safety limit | Pump failure, airlock in heating circuit, low flow rate | Check system pressure (1.0–1.5 bar). Do not reset repeatedly. Call engineer if it returns. | High |
| F4 | No flame detected / ignition failure | Frozen condensate pipe, gas supply issue, faulty ignition electrode, dirty flame sensor | Check outdoor condensate pipe for ice. Thaw with warm water then press Reset (R). If fault returns, call engineer. | High |
| F9 | Fan speed too low | Fan motor failing, blocked flue, poor connection to burner control unit | Do not attempt to reset repeatedly. Flue blockage or fan failure — engineer required. | High |
| Eb | Heat loss during calibration | Ionisation electrode fault, gas valve issue | Technical fault — engineer diagnosis required. Do not continue resetting. | Medium |
| F6 | Fan not at standstill / safety fault | Poor connection to burner control, fan not stopping correctly | Boiler enters emergency mode. Engineer inspection required. | Medium |
| F5 | Flue gas thermostat triggered | Blocked flue, inadequate ventilation, condensate issue | Check external flue outlet for blockage. Press Reset. If fault persists, call engineer. | Medium |
| 0C | Insufficient power supply (100-W) or ghost flame (200-W) | Voltage drop (100-W) or gas valve / ionisation electrode fault (200-W) | 200-W: immediate engineer inspection — possible gas valve fault. 100-W: check electrical supply. | Medium |
| D4 | Internal BUS fault (200-W) or external safety equipment fault (Vitotronic) | PCB communication error (200-W), wiring fault at X2/X3/X7 | Boiler may run 30 min in emergency mode. Engineer required for PCB or wiring diagnosis. | Medium |
| A4 | Burner blocked — pump issue (222-F) | Air lock in internal pump, flow rate below minimum threshold | Check system pressure. If correct, airlock needs engineer to diagnose and clear. | Medium |
| 10 | Outdoor temperature sensor short circuit | Sensor failed or wiring short | Boiler defaults to 0°C safety temperature. System still runs but may under-heat. Sensor replacement needed. | Low |
| 18 | Outdoor temperature sensor lead break | Sensor cable disconnected or broken | Check sensor cable connection. If no visible break, sensor replacement needed. | Low |
| D1 | Outdoor sensor disconnected | Sensor unplugged or cable fault | Check wiring. Boiler may run in constant-mode. Engineer for sensor replacement. | Low |
| 5C–5F | Communication error — multi-boiler BUS | Controller-to-boiler handshake failure (Vitotronic 300-K systems) | Commercial / cascade setup fault. Secondary boilers stop responding. Engineer diagnosis required. | Info |
Stuart’s pro tip for Wokingham homeowners
If your Viessmann boiler shows an F4 or F5 fault after heavy rain or a cold snap, check the external condensate pipe first. These plastic pipes can freeze or become blocked with debris, choking the boiler and triggering a lockout. Thaw the pipe with warm (not boiling) water poured along its length, then press the R (Reset) button for 2 seconds. This clears the fault without a call-out fee in many cases.
Viessmann specialist
Why Viessmann boilers need an accredited engineer
Unlike standard boilers, the Vitodens range uses a high-grade Inox-Radial stainless steel heat exchanger. These are incredibly durable but require specific combustion analysis during any repair to confirm they’re running at their rated 98% efficiency. A standard visual service isn’t enough — and won’t protect your 10 or 12-year warranty.
As a Viessmann Accredited installer, Thermotec carries the specialist diagnostic equipment built for Viessmann’s unique control architecture — enabling precise fault identification rather than parts-swapping, and full combustion re-calibration after every repair.
Common questions
Viessmann boiler FAQ — Wokingham
What does the flashing bell or red indicator mean on my Viessmann boiler?
The bell icon means the internal control unit has logged an error. To see the specific code, press the OK and Menu buttons simultaneously. Once you have the code (e.g. F4 or F2) you can use the table above to determine next steps.
How do I top up the pressure on a Viessmann Vitodens 050-W?
Locate the filling loop — a flexible silver hose with two black handles, usually beneath the boiler. Open both taps slowly until the pressure gauge reads 1.5 bar, then close both taps fully. If you need to do this more than once a month, there’s likely a hidden leak in the system.
What is the most common cause of the Viessmann F4 fault code?
F4 means no flame has been detected. In Wokingham the most common cause in winter is a frozen condensate pipe. Other causes include a faulty ignition electrode, a gas supply interruption, or a dirty flame-sensing probe. Try thawing the external pipe first — if the fault returns, a specialist diagnostic is needed.
How do I reset a Viessmann boiler?
On most Vitodens models, press and hold the R (Reset) button on the front panel for 2 seconds. On touchscreen models a Reset prompt appears on screen. If the boiler fires but the fault returns immediately, stop resetting — repeated attempts can damage ignition components.
Why are my radiators cold even though the Viessmann boiler is on?
Often a faulty outdoor weather compensation sensor (codes 10, 18 or D1) — the boiler thinks it’s warmer outside than it is and reduces output. Could also be a stuck diverter valve or an airlock in the pump. Book a diagnostic to pinpoint the cause.
Does my Viessmann boiler need a special type of service?
Yes. The stainless steel Inox-Radial heat exchanger requires a full combustion analysis, not just a visual check. To keep your 10 or 12-year warranty valid, a Gas Safe engineer must check combustion levels, the burner gasket and ionisation readings at every service. See our Viessmann servicing page.
Related pages
More Viessmann & boiler support
Viessmann fault code you can’t resolve? Call Stuart.
Viessmann Accredited · Gas Safe 599015 · £96 fixed diagnostic · 90% first-time fix · Same-day where available · Wokingham & RG40