Bosch ER4 — Temperature sensor failure — oven cannot read internal temperature

What Bosch ER4 error means on your oven

Error code ER4 indicates that the oven temperature sensor (RTD — Resistance Temperature Detector) has failed or is reading outside the expected range. The sensor is a probe inside the oven cavity that measures temperature in real time, allowing the control board to cycle the heating elements on and off to maintain the set temperature. Without reliable sensor data, the oven cannot control heat safely. If you encounter the bosch er4 error, professional Bosch oven repair may be needed.

Symptoms

• The oven displays ER4 and won’t heat up
• The oven may overheat if the error is intermittent — the board loses temperature feedback and runs the element continuously until it detects the fault
• Cooking results are inconsistent — underdone or overdone food
• The self-clean cycle may not start (the board requires a working sensor for self-clean safety)

Common causes

• Sensor probe failure — the RTD element degrades from years of high-temperature exposure, shifting its resistance values
• Damaged sensor wire — the wire connecting the sensor probe to the control board passes through the oven wall and can be damaged by heat, racks, or during cleaning
• Loose connection — the sensor plug on the back of the oven or at the control board has corroded or come loose
• Self-clean damage — the extreme temperatures during a self-clean cycle (up to 900°F / 480°C) can accelerate sensor degradation

See all Bosch oven error codes for related fault codes and diagnostic steps.

Troubleshooting steps you can try

1. Power reset — turn off the breaker for 5 minutes. A one-time reading glitch can sometimes be cleared.
2. Check the sensor probe — open the oven and locate the sensor probe — a thin metal rod, usually extending from the top or back wall. Check that it is not bent, touching the oven wall, or visibly damaged.
3. Test with a multimeter — disconnect the sensor at its plug (behind the oven or inside the back panel) and measure the resistance. At room temperature, a Bosch oven RTD sensor should read approximately 1,080–1,100 ohms. A significantly different reading confirms failure.
4. Check Bosch support — visit Bosch’s official oven support page for model-specific troubleshooting guides and tech notes.

When to call a technician

The temperature sensor itself is relatively inexpensive and straightforward to replace — it mounts inside the oven with one or two screws and plugs into a harness at the back. However, if the wiring between the sensor and the control board is damaged, tracing and repairing it requires more involved work. If the sensor has been failing intermittently, check the control board relay as well — repeated overheating events can damage it. Codes may vary by model generation and series. Schedule a certified Bosch technician for fast, reliable diagnosis and repair.