The oxygen sensor in a Volkswagen Passat detects the oxygen content of the exhaust gases. It uses that information to change the fuel-air mixture in order to maintain efficient combustion in the engine. Depending on the year in which a special Passat was built, it will have one or more oxygen sensors. The buildings before 1996 have only one, but later models have multiple sensors. There are several symptoms that may indicate a defective oxygen sensor.
Some cars have a special warning light for the oxygen sensor, but on a Volkswagen Passat warning light are either "check engine" or "service vehicle soon" light. There are other issues that can cause these lights to light, but the oxygen sensor is high on the list, especially if other symptoms occur.
A reduction of fuel efficiency can also indicate a problem with one or more oxygen sensors on a Volkswagen Passat. Otherwise, an emission test is another possible symptom. Smelling "rotten eggs" in the exhaust can be caused by a poor oxygen sensor, too. The smell of unburnt gasoline can also indicate an oxygen sensor failure.
Reduced performance may be one of the first symptoms detected when the oxygen sensor is defective. A Volkswagen Passat can get sluggish or hesitated from a poor oxygen sensor. It may stop or be difficult to start, or symptoms may be as subtle as a little less acceleration than normal. Loss of performance, but is a likely symptom of an incorrect oxygen sensor.