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Downstream O2 Sensor or Catalytic Converter?

I had a question recently about replacing an oxygen sensor on a 1997 Mercury Mountaineer. The vehicle failed an emissions test and the code indicated a bad downstream sensor. This can be a tricky diagnosis because if the sensor is reading outside of normal limits, it could be because the sensor is bad, or it could be because the catalytic converter is bad, or both. Certain tests can get to the bottom of this.

First though, let’s review the oxygen sensors, their purpose and function. The oxygen sensor is screwed into an exhaust pipe or manifold so that the sensor’s business end is in the exhaust stream. This sensor is a voltage generator. That is, when hot enough, the peculiar metal compounds in the sensor can produce up to one volt in the presence of oxygen. The vehicle computer uses the information to control fuel. When the sensor reports a rich condition, the computer reduces injector ‘on’ time. When it detects leanness, injector ‘on’ time is increased. This cycle repeats many times per second to maintain optimum fuel to air ratio. It is possible to watch this all happening if one has the right equipment. A graph of the action would show a wave form that, typically, goes from about .2v to .8v.

In about 1994 the U.S. Federal government required cars and light trucks to be able to tell if a catalytic converter is functioning properly. To do this, manufacturers used the same O2 sensors as always, but put them in the exhaust stream behind the converters. If the catalytic converter is doing its job, the sensor reading should be steady, less than .5v, and not cycle like the upstream sensor. When a converter is bad, it no longer converts the excess fuel and oxygen and so the downstream sensor mimics the upstream sensor by cycling and producing a wave form. This is clear evidence of a bad cat converter.

To test a sensor, one can force a rich or lean condition and watch how the sensor responds. If it does respond, then it is probably okay. If you cannot force a sensor to go rich or lean, then it is not functioning.

These tests are not something the average person can do at home. In my experience, a code for a bad sensor usually ends up with a diagnosis of a bad cat converter. Generally, only an experienced dealership mechanic can get to the right answer and so avoid the expense of replacing good parts.

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