Oven
F21
The F21 error code on a Zanussi oven indicates a fault with the oven’s temperature sensor (NTC probe) or its associated wiring — it’s a moderately serious fault that will prevent your oven from heating correctly and should be addressed promptly.
What does this error mean?
F21 means your Zanussi oven’s control board is receiving an out-of-range or implausible signal from the NTC temperature sensor, which monitors the internal cavity temperature. This could mean the sensor is reading temperatures that are impossibly high, impossibly low, or simply not responding at all. Until the fault is resolved, the oven will typically lock out heating to prevent overheating or unsafe operation.
Est. repair cost
£15–£35 DIY sensor replacement; £80–£150 professional repair; £150–£250 if PCB replacement is also required
🔍 Likely causes
🛠️ Step-by-step fix
Power cycle the oven: switch it off at the wall socket or trip the circuit breaker, wait 5 minutes, then restore power. A transient electronic glitch can occasionally trigger F21 and a hard reset clears it.
Inspect the oven interior for obvious damage: look for any visible wiring near the back wall or roof of the oven cavity that appears burnt, melted, or disconnected. Do not touch or attempt to reconnect wiring yourself — this is a job for a professional.
Check the sensor connector (accessible without tools on some models): locate the NTC probe, usually a small metal rod mounted on the back wall inside the oven cavity. Ensure its plug-in connector is firmly seated. If it looks burnt or corroded, stop and call an engineer.
Test the NTC sensor resistance with a multimeter (confident DIYers only): disconnect power, remove the sensor (typically held by one or two screws), and measure resistance across its two terminals. A healthy Zanussi NTC sensor reads approximately 980–1,000 ohms at room temperature (20°C). A reading of 0 ohms (short) or infinite ohms (open circuit) confirms a failed sensor.
Replace the NTC sensor if confirmed faulty: source a genuine Zanussi-compatible NTC sensor (part numbers vary by model — check your model plate inside the door frame). Replacement sensors cost £15–£35 from UK suppliers such as eSpares or 4ourhouse. Fit the new sensor, reconnect the wiring, and restore power to check whether F21 clears.
If F21 persists after sensor replacement, the control board (PCB) is likely faulty — do not attempt to repair the PCB yourself. Book a qualified appliance engineer at this stage.
📞 When to call a professional
Call a qualified appliance engineer if the error persists after a power reset and sensor replacement, if you find burnt or damaged wiring inside the oven, or if you are not comfortable using a multimeter. When booking, tell the engineer: ‘My Zanussi oven is showing error code F21 — I believe it’s related to the NTC temperature sensor or wiring.’
⚖️ Repair or replace?
A professional diagnosis and NTC sensor replacement typically costs £80–£150 all-in, including the call-out. If the control board (PCB) is also faulty, repair costs can rise to £150–£250, at which point replacement becomes worth considering for ovens over 8 years old. For a Zanussi oven under 7 years old in otherwise good condition, repair is almost always the better value — new comparable built-in electric ovens start at around £250–£350.