The oxygen sensor on your vehicle has one primary but important function: it tells your vehicle's computer if the fuel delivery to the engine reads too lean or too rich. The O2 sensor normally works when the engine heats up, and since the sensor works in a closed loop condition like many other sensors on the vehicle, it can monitor the engine's emission control, fuel economy and overall power. Just because an engine light comes on with an emission control warning does not mean the O2 sensor has failed, but that a service reminder interval has expired, requiring your attention. You can check the operation of the O2 sensor in a few easy steps and with a few tools.
Set the transmission shifter in park if you have an automatic; set the shift in neutral for a manual transmission. Set the emergency brake. Raise the car with a floor jack and place two jack stands under the front frame and two stands under the rear frame. Slide under the vehicle and locate the O2 sensor on the main exhaust pipe or exhaust manifold, just before the catalytic converter. You might have more than one sensor; check your vehicle's owner's manual for the locations.
Remove the O2 sensor wire connector from the unit; it will be toward the engine side. Spray penetrating oil on the O2 sensor where it screws into the pipe or manifold. Let it soak. Place an O2 sensor socket over the unit and unscrew it with a ratchet wrench. Be careful not to force it; nudge it tight and back it off several times before loosening it.
Examine the tip of the sensor. It should appear smooth and light grey in colour. Any white, fluffy scale will indicate water contamination. Any heavy, black soot or carbon will not allow the tip probe to read a signal correctly. Any red discolouration or glue-like substance indicates silicone adhesive has covered the tip, which might have been used on a pan or valve cover gasket.
Examine the orifice holes in the body of the O2 sensor up toward the wire connector end and at the probe end. There should be three clean, open holes at both locations. If plugged, use a paper clip to probe the holes open. Clean the O2 sensor with carburettor cleaner and a tooth brush, but do so gently until all scale, mud and debris have been removed. Wipe dry with a rag.
Set a multimeter to the low reading ohms scale. Connect one lead of the multimeter to one black wire coming out of the sensor and the other multimeter lead to the other black wire, which consists of the heating element wires. Do not allow the meter leads to touch any other pin in the connector. Your reading should display about 12 ohms of resistance. A reading that exceeds 12 ohms or reads to infinity means that the sensor has failed.
Place the O2 sensor in a bench vice with the probe (not wire) end sticking out. Connect the multimeter leads to the other two coloured wires in the connector; they might be blue and white or some other colour. Turn the multimeter on the low, 2-volt scale. Use the propane torch to heat up the sensor probe until it glows pink.
Watch the volt scale when applying heat. It should read 1 volt. Pull the torch away and let the senor cool. The reading should drop to 0 (zero) volts. Perform the heating and cooling several times and watch for the 1 volt drop to 0 (zero). Any other reading will indicate a defective O2 sensor. Check and clean all O2 sensors on your vehicle in the same manner.