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Error code reference

Bosch Microwave Error Code E1

Thermistor open circuit detected.

Bosch codesBosch microwave codes

Thermistor open circuit detected.

First step from the service manual

If detected, unit reverts to plug-in mode; display shows error code for 3 seconds. Check thermistor and wiring for open circuit.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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  • Step-by-step tests with expected resistance, voltage, and continuity values
  • Wiring diagram references and connector pinouts from the OEM service manual
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What E1 means on a Bosch microwave

The E1 error code on a Bosch microwave indicates that the control system has detected an open circuit condition in the thermistor. The thermistor is a temperature-sensing resistor responsible for monitoring the internal cavity or component temperatures during operation. When the thermistor circuit reads as open, it means the resistance has risen to an unacceptable level or the circuit has broken entirely, making it impossible for the control board to receive valid temperature data. As a safety response, the unit automatically reverts to a basic plug-in mode and displays the E1 code for approximately 3 seconds before entering this fallback state.

An open circuit in the thermistor can result from several conditions: a failed thermistor component itself, a broken wire in the thermistor harness, a disconnected connector, or corrosion at a terminal point that has interrupted continuity. Thermistors are typically NTC (negative temperature coefficient) devices, meaning their resistance decreases as temperature rises. An open circuit produces an infinitely high resistance reading, which the control board interprets as a critical fault rather than any valid temperature value.

Diagnosing this fault requires inspecting the thermistor and its associated wiring for visible damage, then using a multimeter to confirm continuity and proper resistance values across the thermistor leads. Connectors should be checked for corrosion or looseness, and the wiring harness should be traced for any pinched or broken sections. If the thermistor component itself reads open, it will need replacement to restore normal microwave operation.

Source: manufacturer service documentation.

Common causes of E1

  1. 01

    Failed thermistor component

    Part replacement

    The thermistor itself may have burned out or failed internally, causing it to read as an open circuit. Testing with a multimeter across the thermistor leads will show no continuity or infinite resistance if the component has failed.

  2. 02

    Broken or damaged wiring harness

    Part replacement

    The wires connecting the thermistor to the control board can become pinched, burned, or broken over time, interrupting the circuit. A visual inspection of the harness routing and a continuity test on each wire will identify a break.

  3. 03

    Disconnected or loose connector

    Wiring / connection

    Vibration during operation can cause wiring connectors at the thermistor or control board end to work loose, creating an open circuit condition. Check that all connectors in the thermistor circuit are fully seated and secure.

  4. 04

    Corroded terminal or connector pins

    Wiring / connection

    Moisture or heat exposure inside the microwave cavity can cause corrosion on connector pins or thermistor terminals, increasing resistance to the point of an open circuit reading. Inspect terminals for oxidation or pitting and clean or replace as needed.

See the test procedure for each cause

Frequently asked questions about E1

What does E1 mean on a Bosch microwave?
The E1 code on a Bosch microwave indicates a thermistor open circuit has been detected. The thermistor is the internal temperature sensor, and when its circuit reads as open, the control board cannot receive valid temperature data. The microwave responds by reverting to plug-in mode and displaying the E1 code for 3 seconds. The thermistor itself or its wiring will need to be inspected and tested to resolve the fault.
Can I fix the E1 error on my Bosch microwave myself?
A knowledgeable DIYer can inspect the thermistor wiring and connectors for obvious damage or loose connections, which may resolve the error without parts. However, working inside a microwave involves high-voltage components, including a capacitor that can retain a lethal charge even when unplugged, so the repair should only be attempted by those comfortable with proper high-voltage discharge procedures. If the thermistor component itself has failed, it will need to be replaced, which may require sourcing a compatible part.
Is it safe to use my Bosch microwave when it shows an E1 error?
When the E1 error is active, the microwave reverts to a limited plug-in mode, meaning normal cooking functions are not available. Operating the microwave without a functioning thermistor removes an important layer of temperature monitoring and protection. It is best to avoid using the appliance for cooking until the thermistor fault has been diagnosed and corrected.
Will resetting my Bosch microwave clear the E1 error?
Unplugging the microwave and plugging it back in may temporarily clear the displayed code, but the E1 fault will return if the underlying open circuit condition in the thermistor or its wiring has not been repaired. A reset does not fix the hardware fault, it only clears the stored error state momentarily. The thermistor circuit must be tested and the root cause corrected for the error to stop recurring.
How much does it cost to repair a Bosch microwave E1 thermistor error?
Repair costs vary depending on whether the issue is a failed thermistor, damaged wiring, or simply a loose connector. A replacement thermistor part typically ranges from around $15 to $60 depending on the model. If professional service is needed, labor costs can bring the total repair estimate to roughly $100 to $250. In some cases, a loose or corroded connector can be corrected without any parts cost at all.

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