Cause
Signal wire shorted to the heater circuit
An oxygen sensor's signal wire shorted to its own heater circuit inside the harness or the connector. The sensor produces a signal measured in millivolts and its heater runs on battery voltage, and they travel in the same cable — so a chafe between them puts twelve volts onto a line that should never exceed one.
The module sees an impossible value and reports the sensor, which is usually undamaged. A signal pinned high with no response to mixture changes is the pattern, and it will not move whatever the engine does.
The short is nearly always inside the sensor's own pigtail, where the cable passes close to the exhaust and its insulation hardens and cracks, or at the connector where the wires are crowded. Unplug the sensor and check the signal line at the module end — a voltage still present with the sensor disconnected proves the fault is in the harness rather than the sensor, and that single test decides which part to buy. Repair the chafe rather than only the sensor, or the new one is damaged the same way.
Typically a moderate job.
Codes this can cause (3)
Codes where this is a likely explanation first, rather than merely a possible one.
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