Making sense of oxygen sensors


Here are some diagrams to help make sense of which oxygen sensor and where it is located on your vehicle. These are General Motors specific but most of the manufacturers use the same locations.



A lot of cars are failing our state inspection program for the yellow "SERVICE ENGINE SOON" or "CHECK ENGINE" light in the dash that is lighted up or there is a stored trouble code for "Catalyst Efficiency Low Bank1 Sensor 2" or "Catalyst Efficiency Low". This usually means that the vehicle computer has detected that the catalyst converter is not working as expected. The computer decides this by looking at the upstream oxygen sensor and comparing it to the downstream oxygen sensor. When it pick up that both readings are mostly identical it will set a code and turn on the "SES" light. This does not necessarily mean that cat converter is bad though, it could be a small vacuum leak, slow oxygen sensor, loose electrical connections, leaking oil and engine mechanical condition. I have seen a lot of failures due to oil wicking its way down or up the wiring harness and clogging up the vent hole in the oxygen sensors causing them to stop working.






The way an O2 sensor works is by measuring the difference in oxygen content in the exhaust stream compared to the outside. If there is no difference then the sensor does not generate any voltage and will set a trouble code in memory.



Click on this link for a very informative article out of one of my trade magazines that explains the different types of oxygen sensors and how they work:




Below are some graphic representations to better visualize what the computer is looking at.






As you can see just looking at the numbers does not always show the whole picture, when you look at them with a scope you can see how slow O2S11 is compared to O2S21.












In this picture O2B1S2 is flatlined because the end of the wire harness was soaked with oil and road dirt which plugged up the vent hole.







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Last Updated on 3/15/2006
by Scott Throneberry