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A protective shield or shroud around a sensor, damaged or left off, so the sensor is exposed to conditions it was shielded from. The shield is not cosmetic — it protects the element from direct radiant heat, from the exhaust stream's full velocity, or from water and debris.

A missing heat shield lets a sensor read the exhaust's radiant heat rather than the gas temperature, so it reports high and the module acts on it — cutting fuelling, aborting a regeneration, or reporting an over-temperature that is not happening. A reading that is high and responds too quickly to changes is an unshielded sensor.

They are commonly lost during other work and not refitted, or corroded away on an older exhaust. Check the sensor's installation against the diagram, including any spacer, shield or orientation mark — these details look optional and are not. A sensor replaced without its shield reproduces the fault exactly, which makes this a repeat visit if it is missed.