The F21 error on your Bosch washing machine indicates a fault with the NTC (Negative Temperature Coefficient) thermistor — the temperature sensor that monitors water temperature during the wash and rinse cycles. This error is also sometimes shown as E21 on certain Bosch models. While F21 usually requires component replacement, it is a relatively straightforward repair that can keep your machine running for many more years.
What Does the F21 Error Mean on a Bosch Washing Machine?
The F21 error is an NTC temperature sensor fault. The NTC thermistor is a small resistive sensor mounted on or near the heating element inside the drum housing. It continuously sends temperature data to the control board. If the thermistor’s resistance reading falls outside the expected range — due to a broken sensor, damaged wiring, or a corroded connector — the control board cannot verify the water temperature is safe and halts the cycle with F21. Without reliable temperature data, the machine cannot control the heating element safely.
Common Causes of the F21 Error
- Failed NTC thermistor: The thermistor’s resistive element degrades over time — particularly in machines used frequently on high-temperature programmes. The sensor eventually reads outside its valid range and triggers F21.
- Loose or corroded wiring connector: The wiring harness connector on the thermistor can loosen or corrode from moisture exposure, causing an intermittent open circuit that the board reads as a sensor fault.
- Limescale on the thermistor: In hard water areas, limescale deposits can coat the thermistor, insulating it from the water and causing it to read a lower temperature than actual — triggering F21 as the board expects a temperature rise that does not register.
- Short circuit in the sensor circuit: A damaged wire in the thermistor loom can short to the machine casing, giving a permanently low resistance reading that the board interprets as a failed sensor.
- Control board fault: Rarely, the control board’s temperature monitoring circuit fails and misreads a perfectly good thermistor as faulty.
How to Fix the F21 Error: Step-by-Step
Step 1: Perform a soft reset
Turn off the washing machine and unplug it for 1–2 minutes, then restart. A brief condensation event or power fluctuation can cause the thermistor connector to give a transient false reading, triggering F21. A reset clears this. If F21 does not return, monitor for recurrence over the next few washes.
Step 2: Descale the machine
Run an empty hot cycle (60°C or 90°C) with a washing machine descaler or 500 ml of white vinegar. Limescale coating the thermistor is a common cause of F21 in Indian hard water areas. Descaling monthly prevents sensor misreadings. If F21 appeared after a long period without descaling, this step alone may resolve it.
Step 3: Check the NTC thermistor wiring connector
Unplug the machine. Remove the back panel to access the drum housing and heating element area. Locate the NTC thermistor — it is a small cylindrical sensor clipped onto the heater or drum with two wires attached. Disconnect and firmly reseat the wiring connector. Check the wires for visible damage, kinking, or corrosion at the terminals. Reconnect and test.
Step 4: Test and replace the NTC thermistor
With the machine unplugged, disconnect the thermistor wiring. Set a multimeter to resistance (Ohms). A healthy Bosch NTC thermistor reads approximately 10,000–15,000 Ohms at room temperature (20°C). An open circuit (OL) or a reading far outside this range confirms a failed sensor. A replacement Bosch NTC thermistor costs ₹200–₹600 online and clips into place without specialist tools.
When to Call a Bosch Service Technician
If replacing the NTC thermistor does not resolve F21, the control board’s temperature monitoring circuit may be faulty and needs professional diagnosis. Contact Bosch India customer care (1800 266 1880) or an authorised service centre. Control board replacement is the most expensive repair — typically ₹4,000–₹10,000 including labour.
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Frequently Asked Questions
Can the F21 error appear without an actual fault?
Yes. Condensation on the thermistor connector after a very cold night, or a power surge during a hot cycle, can trigger a false F21. A reset and a descaling run often resolves intermittent F21 errors. If F21 appears consistently on every warm or hot programme, the thermistor needs testing with a multimeter.
How much does it cost to fix the F21 error in India?
A replacement NTC thermistor costs ₹200–₹600 online and is a straightforward DIY replacement. If the control board is faulty, Bosch-authorised repair costs ₹4,000–₹10,000 including parts and labour. A standard service visit call-out charge is ₹500–₹1,500 before parts.
Does Bosch’s warranty cover the F21 error?
Bosch’s 2-year warranty covers NTC thermistor failure due to manufacturing defects. Sensor failure caused by limescale damage from hard water is typically not covered, as Bosch’s user manual recommends regular descaling. If you are within the warranty period and have maintained the machine correctly, contact Bosch India with your purchase details.
Related Bosch Washing Machine Errors
E17 Error (Water Not Filling) | E3 Error (Door Not Closed) | E23 Error (Drain Pump) | F57 Error (Heating Relay)