Temperature sensor
Common error-code part. Confirm resistance and connector style.
F21 on a Bosch washer means the machine started heating the water but couldn’t reach the target temperature before the heating timer expired. The machine paused the cycle rather than continue washing in cold water for a program that specifically requires heat. This is primarily a heating system problem, though sensor faults can cause false F21 errors.
Quick Reference Table
| Code | Meaning | Severity | DIY? |
|---|---|---|---|
| F21 | Heating timeout — water didn’t reach target temperature | Medium | Yes |
Programs That Trigger F21
F21 can only appear on cycles that require heated water:
- Cotton/Normal at temperatures ≥ 40°C (104°F)
- Sanitize or Hot cycles
- Allergy/Hygiene modes
On cold water programs (Cold, 20°C), F21 won’t appear because the heater is never activated.
Step-by-Step Fix
Step 1 — Power Reset
Before opening the machine:
- Unplug the washer for 5 minutes.
- Plug back in and run a 60°C Cotton cycle.
- If F21 reappears, proceed to hardware diagnosis.
Step 2 — Test the Heating Element
The heating element is the most common cause of F21. It sits at the bottom of the drum tub and heats the water that contacts it during the cycle.
- Unplug the washer.
- Pull the machine away from the wall.
- Remove the rear panel (Phillips screws around the perimeter).
- Locate the heating element — it’s a U-shaped metal component at the bottom of the tub with two electrical terminals and a sensor connection.
- Disconnect the wiring connectors.
- Use a multimeter set to resistance (ohms):
- Test across the two main terminals. A healthy element typically reads 20–30 ohms.
- Open circuit (infinite resistance) = burned out element.
- Near 0 ohms = shorted element.
- Also test each terminal to the metal body of the machine (ground test). Any reading other than open circuit indicates the element has grounded and must be replaced.
- To remove the element: loosen the nut on the element body (the central bolt you see between the terminals), then push the element inward slightly to release the locking tabs, and pull it out through the rear opening.
Step 3 — Test the NTC Temperature Sensor
The NTC (Negative Temperature Coefficient) sensor is mounted on or near the heating element and tells the control board the current water temperature. A faulty NTC can cause F21 even when the element is working fine — the board thinks water is cooler than it is and keeps heating, or it gives a wrong reading that stops the cycle prematurely.
- With the element accessible, locate the small sensor (often a cylindrical component clipped near the element with two thin wires).
- Disconnect and test with a multimeter.
- At room temperature (~68°F/20°C), a healthy NTC typically reads 10,000–15,000 ohms on Bosch washers.
- Outside this range or open circuit = failed sensor.
Step 4 — Check the Control Board Relay
The control board has a relay that switches power to the heating element. If this relay fails open, the element never receives power. Symptoms: element and NTC both test fine, but no heat is produced.
You can test this by verifying voltage at the element terminals during a hot cycle (with appropriate safety precautions — this involves live voltage). If 120V/240V (depending on your market) is absent at the terminals, the relay has failed.
Control board replacement is a higher-cost repair — get a firm quote on the board price before committing.
Step 5 — Check Water Supply Temperature
If your home uses tankless hot water or your water heater is set very low, the machine starts with very cold water and the internal heater has to work harder. On heavily insulated Bosch machines in very cold climates, this can occasionally cause borderline F21 events. Raising your water heater temperature can help, though this isn’t the primary cause of most F21 errors.
Parts you may need:
- Bosch Washer Heating Element
- Bosch Washer NTC Temperature Sensor
- Digital Multimeter for Appliance Testing
- Bosch Washer Control Board
Clearing F21
After repair:
- Select a Cotton 60°C or Sanitize cycle.
- Let it run through the heating phase.
- If water heats properly, F21 will not reappear.
Some models require a manual reset: press and hold Spin Speed + Temperature simultaneously for 5 seconds.