How do I test an oxygen sensor?
Short answer: Check the sensor's live data and heater circuit with a scan tool, then confirm its response by watching it switch on a scope or graph while forcing the mixture rich and lean.
A conventional zirconia upstream sensor switches between low and high voltage as the mixture swings lean and rich. On a warm engine in closed loop, graph its voltage. A healthy sensor switches quickly and covers a broad range. A lazy sensor switches slowly or stays in a narrow band.
Test response by adding a controlled amount of propane for a rich condition, or creating a small vacuum leak for a lean one, and watch how fast the signal reacts. Many newer vehicles use wideband air-fuel ratio sensors that report current or a lambda value instead of switching, so read service information for how your tool displays them.
Also check the heater circuit for power, ground and resistance, since heater faults set their own codes. Remember that the sensor reports what it sees; an exhaust leak or misfire can make a good sensor look bad. Work carefully around hot exhaust and use eye protection.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.