Cause
Seized sensor thread in the exhaust
A sensor seized into its exhaust boss by corrosion and heat cycling, which is a repair obstacle rather than a fault in itself — but one worth naming, because it changes what the sensible repair is and it is how a small job becomes a large one.
The risk is the boss, not the sensor. Forcing a seized oxygen or temperature sensor commonly shears it, strips the thread, or twists the boss out of the pipe — and the repair escalates from a part to a welded insert or a new section of exhaust. A sheared sensor with its body left in the pipe is the worst outcome and is entirely avoidable.
Heat and patience are the method. Work on a warm-but-not-hot exhaust so the metals have expanded and separated, use penetrating fluid with time to act, and use the correct slotted socket rather than a spanner on the flats. Anti-seize on the new sensor's threads is the step people skip, and it is what stops the next person meeting the same job — though it must be kept off the sensing element itself, which the manufacturer's instructions will say.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (4)
Codes where this is a likely explanation first, rather than merely a possible one.
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
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