The oxygen sensor in a Volkswagen Passat detects oxygen levels in the exhaust. It uses that information to alter fuel-to-air mix in order to maintain efficient combustion in the engine. Depending on what year a specific Passat was built, it will have one or several oxygen sensors. Those built prior to 1996 have only one, but the later models have multiple sensors. There are several symptoms that can indicate a faulty oxygen sensor.
Some cars have a specific warning light for the oxygen sensor, but on a Volkswagen Passat the applicable warning light is either the "check engine" or "service vehicle soon" light. There are other problems that can cause these lights to illuminate, but the oxygen sensor is high on the list, particularly if other symptoms are present.
Decreased performance may be one of the first symptoms detected when the oxygen sensor is faulty. A Volkswagen Passat can become sluggish or hesitate from a bad oxygen sensor. It may stall or be difficult to start, or the symptom can be as subtle as a little less acceleration than normal. Loss of performance, though, is a likely symptom of a faulty oxygen sensor.
A decrease in fuel efficiency can also indicate a problem with one or more of the oxygen sensors on a Volkswagen Passat. Failing an emissions test is another possible symptom. Smelling "rotten eggs" in the exhaust could be caused by a bad oxygen sensor, too. The smell of unburnt gasoline can also indicate an oxygen sensor failure.