ترک یا آسیب بدنه سنسور اکسیژن

The sensor's housing or ceramic element cracked, so it no longer measures a sealed sample of exhaust gas. These sensors compare exhaust oxygen against a reference of outside air, and the reference reaches the element through the sensor body — so a crack lets exhaust and outside air mix and the comparison becomes meaningless.

The failure reads as a permanently lean exhaust. Extra air reaching the element makes the sensor report more oxygen than is present, so the module adds fuel to correct a lean condition that does not exist, and the engine genuinely runs rich as a result. A rich-running engine with a lean-reporting sensor is the signature, and it is the reverse of what people expect.

Thermal shock and physical damage are the causes. Water striking a hot element cracks it instantly, and the sensor's exposed position under the vehicle means stone impact and overtightening during fitting. Inspect the body and the vent for damage before condemning the circuit, and compare against the downstream sensor — two sensors reporting incompatible mixtures locate the faulty one immediately.