What E4 Means
E4 on a Bosch built-in speed microwave oven is the shorted thermistor fault, the counterpart to the open-circuit version. The service manual gives the same behaviour: the oven reverts to plug-in mode and shows the code for three seconds. A shorted sensor reads as an impossibly high temperature, so convection and combination functions are disabled rather than allowed to run blind.
Common Causes of E4
- Thermistor internally shorted
- Sensor wiring shorted against cavity metalwork
- Input module misreading the sensor circuit
🔧 One Safe Check You Can Try
A two-minute reset at the breaker is worth one try. Beyond that, leave the appliance closed: the high-voltage capacitor holds a charge after disconnection and discharging it safely is not a household task.
⚠ Why You Shouldn't Go Further Than That
- A microwave's high-voltage capacitor can retain a lethal charge after the appliance is unplugged, which rules out casual investigation.
- The sensor wiring runs close to the magnetron compartment, the part of the appliance that is least safe to work around.
- Bosch's plan checks the input module as well as the sensor, and misdiagnosing sends you to the more expensive of the two.
- Built-in models are held in cabinetry by a trim kit, and removing one badly damages the surrounding units.
Opening a panel to inspect or repair something yourself doesn't automatically cancel your manufacturer's warranty — but it can risk accidental damage, and some manufacturers do have specific service conditions. Check your product's warranty terms, or ask us, before deciding.