Oven
F21
The F21 error code on a Siemens oven indicates a fault with the temperature sensor (NTC sensor) or its circuit — it’s a moderately serious fault that will prevent your oven from heating correctly or at all.
What does this error mean?
F21 signals that your Siemens oven’s control board has detected an abnormal reading from the oven cavity temperature sensor (also called an NTC or thermostat probe). This usually means the sensor is reading a temperature that is implausibly high, low, or erratic — outside the range the oven expects. Until the fault is resolved, the oven will typically lock out heating functions as a safety precaution.
Est. repair cost
£15–£40 DIY sensor replacement; £80–£180 for professional repair (sensor or PCB)
🔍 Likely causes
🛠️ Step-by-step fix
Power cycle the oven: switch it off at the wall socket or circuit breaker, wait 5 minutes, then switch it back on. This clears temporary control board glitches and costs nothing.
Check the oven cavity for the temperature sensor — it’s typically a thin metal probe, 3–5 cm long, mounted on the back wall inside the oven. Inspect it visually for obvious damage, burning, or corrosion.
Gently press the sensor’s wiring connector to ensure it is firmly seated. If accessible without disassembling panels, reseat the connector and restart the oven to see if the code clears.
Clean around the sensor probe with a damp cloth (oven completely cold and unplugged). Grease build-up can interfere with readings — do not use abrasive cleaners on the probe itself.
Test or replace the NTC sensor: a replacement Siemens-compatible NTC sensor costs £15–£40 online (search your model number on sites like eSpares or 4ourhouse). With the oven unplugged, unscrew the sensor, unplug its connector, and fit the new one — this is a straightforward DIY task requiring only a screwdriver.
If the error persists after sensor replacement, stop DIY work — the fault likely lies with the control board or internal wiring, which requires a qualified engineer.
📞 When to call a professional
Call a qualified appliance engineer if the F21 code returns after a power cycle and sensor replacement, or if you find burnt or damaged wiring inside the oven. Tell the engineer: ‘My Siemens oven is displaying an F21 fault code — I believe it’s an NTC temperature sensor or PCB issue.’ Avoid attempting to open the control board housing yourself, as this involves live electrical components.
⚖️ Repair or replace?
A professional repair for an F21 fault — typically sensor replacement or PCB work — will cost £80–£180 in the UK including parts and labour. If your Siemens oven is under 8 years old and a mid-range or premium model, repair is almost always worth it. For ovens over 10 years old or budget models, compare the repair quote against a replacement oven (entry-level built-in ovens start from around £250) before committing.