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Oxygen Sensors

The oxygen sensor was developed in the late 1960’s by the Robert Bosch Corporation. Porsche first installed an oxygen sensor, often referred to as an O2 sensor, in all their cars starting from 1980. The O2 sensor plays a critical role in the proper performance of the engine and helps support the effective operation of the emissions system. The O2 sensor is physically located in the engine’s exhaust system and monitors oxygen content in the exhaust gases exiting the engine.

The O2 sensor operates by the principle of a chemical reaction that generates a voltage when oxygen in the exhaust gases comes in contact with the precious metals that comprise the O2 sensor. If little or no oxygen is present in the exhaust gases, a rich running condition exists and the voltage will build up higher in the O2 sensor. A lean running condition will have a lot of oxygen in the exhaust stream and this will generate a small amount of voltage. This voltage signal is sent to the Digital Motor Electronics (DME) control unit where the O2 sensor signal is analyzed. The DME then adjusts the fuel delivery to the engine accordingly via the pulse width of the fuel injector on time (Ti). If the O2 sensor indicates a rich mixture then fuel delivery is reduced, with a lean condition fuel delivery is increased.

For O2 sensors to function properly, they need to be at least 500 degrees Fahrenheit. As an O2 sensor ages, it will lose speed and accuracy of operation. When they lose speed, this can cause higher Oxides of Nitrogen (NOx) and in general cause an increase in tail pipe emissions, poor performance and lower fuel mileage due to inaccurate information being provided to the DME. The life of the O2 sensor can be affected by the mechanical condition of the engine; if your engine is burning a lot of oil the O2 sensor will get contaminated. Usually, the most common cause for failure is because of contamination with oil, coolant, silicone or carbon build up.

Testing O2 sensors can be difficult because many diagnostic tools deliver inconsistent results. Generally, cars equipped with On Board Diagnostic-II (OBD-II), 1996 and newer, have improved self diagnostic capability of the O2 sensors but this also cannot always be relied on. A Digital Storage Oscilloscope (DSO) or Graphing Multi-Meter is the best tool to utilize when testing an O2 sensor by means of measuring its voltage range and reaction time. To test the sensor, you drive (or force) the sensor lean to rich and then rich to lean while monitoring how long it takes for the sensor to react from one extreme to the other. A good sensor should not exceed 100ms or 1/10th of a second for each segment test.

When an O2 sensor ages it is referred to as lazy because the sensor’s response time has slowed significantly. When this happens, your Porsche may not pass emissions testing and could possibly run poorly. At that point replacement of the O2 sensor is the necessary corrective action. There is no standard in mileage when replacement of the O2 sensor should be performed but a good replacement interval would be every 60k miles.
Tony Callas & Tom Prine

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