Bosch error code E4: The wash water temperature exceeded the safe operating limit. The machine stops to protect components from heat damage. This is most commonly caused by a failed NTC thermistor that sends incorrect readings to the control board.
The wash water temperature exceeded the safe operating limit. The machine stops to protect components from heat damage. This is most commonly caused by a failed NTC thermistor that sends incorrect readings to the control board.
Disconnect power. Access the heating element (see your model's service manual). Disconnect the heater leads and test with a multimeter set to resistance. A good element reads 10–50 Ω. An open reading (infinite Ω) means the element is burned out.
Tools needed: Multimeter, Screwdriver set
With leads disconnected, test between each heater terminal and the appliance chassis. Any reading other than infinite Ω means the heater has a ground fault and must be replaced immediately.
Tools needed: Multimeter
Most heaters are protected by a thermal cutout. Test the thermostat for continuity — it should read close to 0 Ω at room temperature. A tripped (open) thermostat will prevent any heating. Some thermostats are resettable; others must be replaced.
Tools needed: Multimeter
Mineral deposits on heating elements insulate the surface, cause hot spots, and burn out the element prematurely. On visible elements, look for white/gray calcite crust. Consider running a descaling cycle or using a citric acid treatment.
If the element and thermostats test good, the relay on the control board that switches the heater may have failed. This requires testing the relay coil and contacts, or replacing the board.
Tools needed: Multimeter
Test the NTC thermistor resistance first. If the thermistor is within spec, the heater relay on the control board has likely failed.