Bosch ranges have a reputation for precise heat and sleek control logic. When Er1 flashes on the display, the range is telling you it can’t trust the temperature it’s reading. In plain terms, the oven’s temperature feedback loop is off—usually because of the temperature sensor (also called an RTD/NTC probe), its wiring or connector, or, less commonly, the control module that interprets the sensor’s signal.

What the code is really saying

Your oven decides how long to heat based on a tiny change in electrical resistance from the sensor. If that signal looks impossible—open circuit, shorted, or wildly out of range—the control board throws Er1 to prevent runaway heating or stone-cold baking. That’s a safety feature, not a bug.

Common real-world triggers

Sometimes the sensor has simply aged out after years of high-heat cycles. Other times, a loose plug at the back of the cavity, a nicked wire from a past move, or heat stress at the connector is enough to confuse the board. Rarely, a failing control board misreads a healthy sensor and causes the same code.

Quick checks you can do safely

First, power matters. Turn the range off at the breaker for a full minute, then restore power; this clears memory noise and resets the relay logic. If your model uses a meat probe, make sure it’s not inserted or partially seated when it shouldn’t be—an in-between state can mimic a bad temperature reading. Let the oven cool completely and try a low-temp bake cycle to see if the error reappears right away or only at higher temperatures.

When to stop and call a pro

Past the basics, accurate diagnosis needs a multimeter and manufacturer specs. A tech will measure the sensor’s resistance at room temperature and again when warmed, inspect the harness for intermittents, and confirm continuity right up to the control board. If numbers don’t match the spec or the reading drifts as you wiggle the harness, the fix is usually a new sensor; if the sensor checks out perfectly, attention shifts to the board.

Why DIY can get tricky here

Replacing the temperature sensor sounds simple, but access can mean pulling the range, removing rear panels, and routing the new lead without pinching it against the liner—plus re-seating the gasket so heat and steam don’t leak. Board work is even more delicate: static precautions, correct part revision, and careful reassembly so you don’t create a new fault while solving the old one.

Good news about parts and outcomes

The temperature sensor is a relatively straightforward, affordable component on most Bosch ranges, and swapping a tired one often brings the oven right back to accurate, steady temperatures. Using the correct OEM part matters—fit and resistance curves need to match your model so the control logic can “trust” the reading again.

Preventing a repeat

A clean, well-ventilated cavity and gentle treatment during high-heat cycles can extend sensor life. Avoid slamming pans against the rear wall, don’t tug on the sensor capillary, and let the oven cool with the door closed after self-clean or high-temp bakes so the wiring isn’t shocked by sudden cold air.

Er1 isn’t a mystery as much as a guardrail. It’s your range saying, “I don’t have a believable temperature—help me out.” Rule out the simple stuff with a full power reset and probe check; after that, a sensor-and-harness test by an experienced technician is the fastest path to a clean fix and reliable baking temps again.