Bosch Microwave Error Code E4

E4 on a Bosch microwave indicates that a short circuit has been detected in the thermistor circuit. The thermistor is a temperature sensor that the microwave uses to monitor internal heat levels. When a short is detected, the unit reverts to a limited plug-in mode and displays the E4 code for approximately 3 seconds. The thermistor and its associated wiring should be inspected and tested to identify the source of the short.

When E4 is active, the microwave reverts to a basic plug-in mode, which limits its normal cooking operation. Because the temperature monitoring circuit is not functioning correctly, continued use risks unmonitored internal temperatures. It is best to avoid using the microwave for cooking until the thermistor fault is diagnosed and repaired.

Repair costs vary depending on the root cause. If only the thermistor needs replacement, parts costs are generally modest, typically in the range of $15 to $50 for the component, plus labor if a technician performs the work. If the control board or a larger wiring harness is involved, total repair costs including labor could range from $100 to $300 or more. Getting a diagnostic estimate before authorizing repairs is advisable.

A knowledgeable DIYer can inspect the thermistor wiring harness for visible damage, check connectors for corrosion, and use a multimeter to test thermistor resistance. However, working inside a microwave involves high-voltage components, particularly the capacitor, which can retain a dangerous charge even when unplugged. If you are not experienced with appliance repair, having a qualified technician perform the diagnosis and replacement is strongly recommended.

Unplugging the microwave and plugging it back in may temporarily clear the E4 display, but the error will return if the underlying short circuit in the thermistor circuit has not been repaired. A reset does not fix the component or wiring fault causing the code. Proper diagnosis and repair of the thermistor circuit are necessary for a lasting resolution.