Your vehicle's O2 sensor plays a significant role in its performance and if it's faulty, it's not something you should ignore ...

Bolted directly to the engine block, the exhaust manifold is the first section of a vehicle’s exhaust system ... in erroneous readings by the oxygen sensor, triggering a check engine light.

A lean air/fuel mixture burns slower and not as hot, so some fuel may not fully burn during the combustion cycle, eventually igniting in the exhaust system. A bad oxygen sensor, mass air flow ...