Oven
F21
The F21 error code on a Bosch oven indicates a fault with the oven’s temperature sensor (NTC sensor) or its associated wiring — it’s a moderately serious fault that will prevent the oven from heating correctly or at all.
What does this error mean?
F21 tells you that the oven’s control board has detected an abnormal resistance reading from the NTC (Negative Temperature Coefficient) temperature sensor. This sensor monitors the internal oven temperature in real time and sends data back to the control board. When the reading falls outside the expected range — either too high, too low, or absent entirely — the oven throws the F21 code and shuts down heating as a safety precaution.
Est. repair cost
£15–£40 DIY (replacement NTC sensor); £80–£150 professional repair (sensor + labour); £150–£250+ if PCB replacement is required
🔍 Likely causes
🛠️ Step-by-step fix
Step 1 — Hard reset the oven: Switch the oven off at the wall socket (or trip the circuit breaker), wait 5–10 minutes, then restore power. If F21 clears and doesn’t return, a transient glitch was the cause. Monitor for recurrence.
Step 2 — Check the oven cavity for visible damage: With the oven completely cool and unplugged, look inside for a small metal probe (usually at the back wall or roof of the cavity) — this is the NTC sensor. Check for visible burn marks, grease coating, or physical damage to the probe or its cable.
Step 3 — Clean around the sensor probe: If the sensor is coated in grease or food residue, carefully clean the probe and the surrounding area with a damp cloth and mild degreaser (oven cold and unplugged). Repower and test.
Step 4 — Test the NTC sensor resistance with a multimeter (DIY-confident users): Unplug the oven. Disconnect the sensor’s wiring connector and set your multimeter to resistance (Ω). At room temperature (~20°C), a healthy Bosch NTC sensor typically reads around 4,000–6,000 ohms. A reading of 0 (short) or OL/infinite (open circuit) confirms a failed sensor.
Step 5 — Replace the NTC temperature sensor: Bosch NTC sensors for common oven models are available from UK appliance parts suppliers (e.g. 4Trade, Espares, DGES) for £15–£40. The sensor is usually secured by one or two screws and a wiring connector — straightforward to swap with the oven unplugged. Ensure you match the part number to your specific model.
Step 6 — If the new sensor does not clear the fault: The fault likely lies in the wiring harness or the main PCB. Do not attempt to repair the control board yourself — book a qualified appliance engineer at this point.
📞 When to call a professional
Call a qualified appliance engineer if replacing the NTC sensor does not clear the F21 code, if you find burnt or melted wiring inside the oven, or if you’re not comfortable working with electrical components. When booking, tell the engineer: ‘My Bosch oven is showing an F21 error code — I suspect an NTC temperature sensor or wiring fault.’ This helps them arrive with the right parts.
⚖️ Repair or replace?
A professional repair for F21 — covering diagnosis, a replacement NTC sensor, and labour — typically costs £80–£150 in the UK. If the fault turns out to be the main PCB, expect £150–£250+. For an oven over 8–10 years old, weigh repair costs against a new built-in oven (£300–£700 for a comparable Bosch model). If repair exceeds 50% of replacement cost on an older appliance, replacing is the smarter financial decision.