The oxygen sensor eventually needs to be replaced in all vehicles. But how would you know if the oxygen sensor has gone bad? Fortunately there are ways oxygen sensor testing can be done using OBD2 code reader.

The oxygen sensor or O2 sensor as it is commonly called plays an important role in monitoring the oxygen content of the car’s exhaust system. It determines the optimal mixture of air to fuel ratio needed to run the vehicle. A defective sensor can greatly affect your vehicle’s fuel consumption because it allows the vehicle to use more fuel than what it should. Hence faulty sensors need to be diagnosed and replaced promptly.

But determining if the O2 sensor is really defective will need accurate diagnosis. Equipments like the OBD2 code reader can be used for this purpose. OBD stands for On Board Diagnostics. OBD2 is a diagnostic system, which monitors the performance of vehicles by performing tests associated with its specific systems or components. The 1990 Clean Air Act (CAA) made it mandatory for all vehicles sold in USA to have some form of diagnostic system. Hence all light duty vehicles from 1996 onwards were manufactured with OBD feature.

The emission testing done by OBDII is more comprehensive than the tailpipe test because unlike the traditional tailpipe test, which provides the emission results of only a specific moment (only during testing), the OBD2 code reader can monitor the vehicle emission at various moments when the vehicle is in operation.

The first generation of OBD, called as OBDI provided a simplistic test to determine the health of the oxygen sensor. Oxygen sensor testing using OBD1 involved determining if the sensor was stuck lean or rich for 30 seconds or more. The OBDI systems didn’t have any way to determine the response time of the oxygen sensors.

Oxygen sensor testing using OBD2 does look at the response time as well. A good upstream oxygen sensor produces an oscillating waveform that toggles between 0.1V to 0.9V. The sensor indicates 0.1V for lean mixture and 0.9V for rich air to fuel mixture. So the new OBD2 equipments can test how fast or slow the oxygen sensor transitions between lean and rich mixture levels and hence determines the response time of the oxygen sensor with known "good" values. Good OBD2 scan tools will also display the oxygen sensor output waveform as seen using an oscilloscope.

The response time of oxygen sensor may vary from 0.3 sec to 0.125 sec (depending on the oxygen sensor specification). Slower response time could be an indication of deposition of carbon or fuel on the sensor. Faster switching of the oxygen sensor output can cause misfiring. Emission problems related to engine misfire will make the OBDII lamp to flash. A P0300 code will be set in the event of an engine misfire. Further diagnosis is needed to determine why the cylinder is misfiring.

Another diagnosis that can be done using OBD2 system is determining the efficiency of catalytic converter. The output of downstream oxygen sensor, located after the catalyst is compared with the upstream oxygen sensor located before the catalytic converter. In an efficiently working catalytic converter, the upstream oxygen sensor will switch between lean and rich voltages, but the downstream oxygen sensor will have much fewer voltage switching. If the converter efficiency is poor (due to contamination), the downstream O2 sensor output will look like the upstream O2 sensor output; that is the post cat oxygen sensor will switch more frequently between lean and rich voltage levels just like the pre cat oxygen sensor.

The oxygen sensor faults detected by OBDII system are represented as "codes". Codes can be generic (in which case it will start with prefix P0) or it can be enhanced (such as the OEM specific codes starting with P1). All the OBDII code readers will be able to read all generic codes, but may or may not read all OEM specific enhanced codes.

Oxygen sensor testing using OBD2 can also provide us insights as to which sensor is not functioning properly. Usually the error codes of the OBD2 system can be traced to specific bank number and oxygen sensor number of a vehicle.

So, troubleshooting oxygen sensor problems using OBD2 has become an industry standard as it provides a more reliable and easy way to identify oxygen sensor faults.