Sondenheizung defekt

The built-in heater that brings an oxygen sensor to operating temperature has failed or weakened. The heater is not a warm-up convenience — it is a precondition. A zirconia sensor produces no usable signal below roughly 300°C, and a wideband air-fuel sensor produces nothing meaningful at all until it is hot.

Which is why a heater fault sets codes that look like sensor faults. Closed-loop entry times run long, responses read slow in both directions, and the mixture control loop never starts — all reported against a sensing element that is perfectly good and merely cold. Where a heater code is stored alongside a response or closed-loop code, the heater is the fault and the rest are consequences.

The supply is worth testing before the sensor is condemned. These heaters draw real current to reach temperature in seconds, so a tired battery, a poor earth, a blown fuse or high resistance in the heater circuit slows warm-up enough to fail the test with the element itself intact. Heater circuits also fail at the connector rather than in the wire, because the current heats a marginal terminal, oxidises it further, and eventually burns it — so look for discoloured or melted plastic rather than just checking continuity.