When your Bosch oven flashes Er4, it’s your range raising a hand to say, “I can’t trust my temperature reading.” In plain terms, the oven’s temperature sensor (the probe that reports heat back to the control) isn’t giving a reliable signal. Without that feedback loop, the control board can’t hold a steady setpoint—so preheats stall, roasts come out underdone, and bakes turn patchy or inconsistent.

Why it happens: Most often, the sensor itself has failed or the wiring to it has loosened or degraded from years of heat cycles. Less commonly, the control board can’t read the sensor properly because of a damaged connector or a cooked trace. The end result looks the same to you: the oven tries to heat, realizes the data doesn’t add up, and posts Er4.

How it shows up in the kitchen:

You may notice the oven overshooting or never quite reaching temperature, uneven browning on trays that used to bake perfectly, or long, frustrating preheat times followed by the error. Because baking needs precise, stable heat, bread and pastries are usually the first victims. And while an Er4 code doesn’t mean the oven is unsafe by default, it does mean the control system doesn’t trust its own readings—so it’s something to address promptly.

A couple of simple checks you can try:

Power the oven off at the breaker for a few minutes and restore power; sometimes a clean restart clears a transient fault. If you can easily see the sensor inside the cavity (a small metal probe at the back wall), make sure it’s firmly seated and not bent. Beyond that, diagnosis moves into testing continuity, verifying the harness, and checking the sensor’s resistance against Bosch specs—work best done with the right tools and a cool appliance.

Why professional service helps:

A proper Er4 fix starts with measuring the sensor and wiring, not guessing and replacing parts. A trained tech will verify the sensor reading at room temperature, inspect connectors for heat damage, check the harness where it passes close to hot panels, and only then call it for a sensor or control board. Done right, your oven returns to accurate preheats and stable, even heat—exactly what makes Bosch baking feel effortless.