An oxygen sensor is part of an engine's management system and monitors the amount of oxygen present in the engine's exhaust. The engine's computer uses this information to adjust the air-to-fuel ratio, keeping the oxygen level in the exhaust at an acceptable level for the catalytic converter.
Damaged and worn-out air hoses connecting to the intake manifold can create a lean air/fuel mixture reading at the oxygen sensor. These leaks allow excess air into the engine that the computer cannot control or regulate.
A blown head gasket occurs when the gasket that seals the combustion chamber leaks. As the piston moves down, it sucks in extra air, which gets sent through the exhaust. The oxygen sensor will detect this extra air and indicate a lean air/fuel mixture.
A bad fuel injector will not deliver the proper amount of fuel to the engine. If the injector fails completely, no fuel will be injected into the combustion chamber. This allows a lot of fresh air, with no fuel, to pass through the engine into the exhaust system, where the oxygen sensor will detect a lean mixture because of the excess oxygen.
