How to Test Oxygen Sensors

Updated February 21, 2017

The oxygen sensor is mounted in the exhaust pipe just forward of the catalytic converter. This sensor monitors the amount of oxygen in the exhaust. The sensor sends a voltage signal to the vehicle's on-board computer, which allows the computer to make corrections in the fuel air mixture. Exhaust emissions, fuel economy and engine performance are all affected by the oxygen sensor. If your vehicle has been running poorly, the oxygen sensor may be to blame---follow the steps below to find out.

Operate vehicle until engine reaches normal operating temperature. Gain access to the pre-converter oxygen sensor. Jack and raise vehicle if necessary to reach the sensor, paying attention to safety precautions.

Remove the oxygen sensor connector cover and check the oxygen sensor's operational voltage. Consult your vehicle's manual to find the wiring schematic. Back probe the sensor's connector with the meter leads. Start engine again and monitor voltage readings. Voltage should switch between high and low, from approximately 0.1 to 0.9 vdc. Replace sensor if it fails this test.

Unplug the sensor connector and test the sensor's heating circuit. Using your vehicle's wiring schematic, check for 12 vdc and also for a good ground. If voltage or a ground is not present, repair circuit between connector and battery.

Measure internal resistance of the sensor's heater circuit. Check that the heater resistance is a low reading. Replace sensor if the heater resistance is infinity or extremely high.

Unplugging the sensor's connector while testing the oxygen sensor will often set fault codes and illuminate the "Check Engine" light. After all checks and repairs have been made, clear the fault codes by following the procedure in your vehicle's manual.


Wear gloves to keep from getting burnt on the hot exhaust pipes.


Use caution when working under cars.

Things You'll Need

  • Multimeter with test leads
  • Mechanic hand tools
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About the Author

Daniel Ray has been writing for over 15 years. He has been published in "Florida Sportsman" magazine. He holds an FAA airframe and powerplant license and FCC radiotelephone license, and is also a licensed private pilot. He attended the University of South Florida.