Boiler
F02
The F02 error code on a Viessmann boiler indicates a short circuit in the flow temperature sensor (NTC sensor), and it will lock out your boiler until the fault is resolved — meaning no heating or hot water.
What does this error mean?
Your Viessmann boiler constantly monitors water temperature using an NTC (negative temperature coefficient) sensor on the flow pipe. When the boiler’s control board detects that this sensor is returning a short-circuit signal — rather than a normal resistance reading — it triggers the F02 fault and shuts down as a safety precaution. This is not a nuisance lockout; the boiler is protecting itself and your home from potential overheating.
Est. repair cost
£80–£180 for NTC sensor replacement; £250–£450 if PCB replacement is required
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Reset the boiler: Press and hold the reset button (usually marked with a flame and cross symbol) for 3–5 seconds. If F02 clears and the boiler fires up normally, monitor it closely for recurrence.
Step 2 — Check the boiler manual: Confirm F02 is listed as a flow sensor short circuit for your specific Viessmann model (e.g. Vitodens 050-W, 100-W, 200-W), as fault code definitions can vary slightly between ranges.
Step 3 — Do not attempt further DIY: The NTC sensor, wiring, and PCB are internal gas appliance components. Replacing or testing these requires the boiler casing to be removed and electrical diagnostics to be performed — this must only be done by a Gas Safe registered engineer.
Step 4 — Turn off the boiler if it repeatedly locks out: Continuous lockout attempts without resolving the fault can stress the PCB. Switch the boiler off at the programmer or isolator switch and call an engineer.
Step 5 — Note the fault details before calling: Write down the exact error code (F02), how long it has been showing, whether it appeared suddenly or after a recent service, and your boiler model number (found on the data plate inside the front panel). This helps the engineer diagnose faster and may reduce call-out time.
📞 When to call a professional
Call a Gas Safe registered engineer as soon as the reset does not clear the fault or if F02 returns within a short period. Tell them your boiler is showing fault code F02, which you understand to be a flow NTC sensor short circuit — this immediately points them to the right component. Do not attempt to open the boiler casing or touch any internal wiring yourself, as this is illegal for unqualified persons on a gas appliance and potentially dangerous.
⚖️ Repair or replace?
An NTC sensor replacement typically costs £80–£180 all-in (parts and labour), while a PCB replacement — if that is the root cause — can range from £250–£450. If your Viessmann boiler is under 10 years old and otherwise well-maintained, repair is almost always the right call. If it is over 12–15 years old with a history of faults, a new boiler (£2,000–£3,500 installed) may be more economical over the next 5 years.