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Error code reference

Bosch Microwave Error Code E4

Thermistor short circuit detected.

Bosch codesBosch microwave codes

Thermistor short 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 short circuit.

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

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  • Wiring diagram references and connector pinouts from the OEM service manual
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What E4 means on a Bosch microwave

The E4 error code on a Bosch microwave indicates that the control system has detected a short circuit condition in the thermistor circuit. The thermistor is a temperature-sensitive resistor used to monitor internal cavity or component temperatures during microwave operation. When the thermistor's resistance drops to near zero or falls outside the acceptable range due to a short, the control board interprets this as a fault condition and logs the E4 code.

When E4 is triggered, the unit does not shut down completely. Instead, it reverts to a basic plug-in mode, limiting functionality to protect the appliance from potential thermal damage caused by unmonitored temperatures. The error code displays on screen for approximately 3 seconds before the unit settles into this reduced operating state. This behavior is a protective response, not a hard lockout.

The short circuit can originate from the thermistor component itself, whose resistance has collapsed internally, or from a wiring issue in the harness connecting the thermistor to the control board. Physical damage to the wiring, pinched insulation, moisture intrusion, or a failed thermistor are the most common sources. Diagnosis should begin with a visual inspection of the thermistor and its wiring for obvious damage, followed by resistance testing of the thermistor to confirm whether the component has shorted internally.

Source: manufacturer service documentation.

Common causes of E4

  1. 01

    Failed thermistor (internal short)

    Part replacement

    The thermistor itself can fail internally, causing its resistance to drop to near zero and triggering the short circuit detection. This is the most common cause and requires replacing the thermistor component.

  2. 02

    Damaged or pinched wiring harness

    Part replacement

    The wiring connecting the thermistor to the control board can become pinched, frayed, or worn through, creating a short between conductors. Inspect the full length of the harness for visible damage or contact points with sharp edges.

  3. 03

    Moisture or condensation in the thermistor circuit

    Wiring / connection

    Moisture intrusion near the thermistor connector or along the wiring harness can create a low-resistance path that mimics a short circuit. Check connectors and surrounding areas for signs of moisture or corrosion.

  4. 04

    Loose or corroded connector at the control board

    Wiring / connection

    A corroded or poorly seated connector at the control board end of the thermistor circuit can cause intermittent or sustained resistance anomalies. Disconnecting, inspecting, and reseating the connector may resolve the fault if corrosion is minor.

  5. 05

    Control board fault

    Part replacement

    In rare cases, the control board itself may misread the thermistor circuit due to a failed input circuit on the board. This is typically diagnosed only after confirming the thermistor and wiring both test within normal parameters.

See the test procedure for each cause

Frequently asked questions about E4

What does E4 mean on a Bosch microwave?
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.
Can I fix the E4 error on my Bosch microwave myself?
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.
Is it safe to keep using my microwave when the E4 code appears?
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.
Will resetting my Bosch microwave clear the E4 error?
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.
How much does it cost to repair a Bosch microwave E4 error?
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.

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