The sensor works as a result of the varying quantities of oxygen in the exhaust verses the amount in the atmosphere. Voltage is produced by the difference in the two amounts. If the amount of oxygen in the exhaust is closer to the amount in the air, the engine is lean and the voltage is low (under 250mv). If the engine is rich, the voltage is high (about 950mv).

Natural aging, shock from accidents, antifreeze poisoning, excessive oil consumption or leakage, silicone poisoning due to incorrect use of silicone gasket sealers, etc.