An oxygen or O2 sensor measures the amount of oxygen in the exhaust gases of a vehicle. The automobile's computer system uses the information gathered by the sensor to control the engine operation. When something goes wrong with the oxygen sensor, a check engine light is generally activated. However, when the sensor is just beginning to go bad, it may cause problems without activating a light.
Rough Engine Idle
A bad O2 sensor can cause an engine to run rough or irregular because they control many different engine functions. A faulty sensor can cause a disruption in the fuel/air mixture, engine timing or engine combustion intervals. The car senses an O2 sensor malfunction and injects more fuel than it needs to prevent engine trouble. The extra fuel results in a stall which causes the rough engine idle.
A engine miss, or irregularly running engine, can be caused by a bad air/fuel mixture or incorrect engine combustion. It also can cause a problem in the fuel delivery/combustion inside an engine. Each of these problems can be caused by a bad O2 sensor. It is easiest to diagnose at idle or at lower engine speeds.
Poor Gas Mileage
Typical gas mileage is disrupted when the automobile's fuel-delivery and fuel-combustion systems are made irregular by a bad O2 sensor. If the systems are thrown off, then too much fuel can be injected into an engine's cylinders or the air/fuel mixture of the engine is disturbed. The automobile injects more fuel than needed when the sensor fails and causes more gas usage than normal.
Increased Vehicle Emissions
The oxygen sensors of a vehicle help to keep the automobile's emissions within certain limits. When the O2 sensor fails, the vehicle can output smog because the sensor is not accurately measuring the air/fuel mixture and does not adjust engine operations accurately enough to keep these emissions in the proper ratio.