To check a O2 sensor a digital multimeter is fine. When its a single wire sensor its a snap. Carefully insert a small paper clicp or something like it from the back of the connector connect positive wire of meter grount the other one
and you are good to go. On 3 wire sensor one wire is ground one is 12 volts with the ignition on and one is the sensor wire 0.0 to 0.8 volts. If you have a scanner you can also see if the feedback system works by driving it lean and rich and look for the fuel numbers to change, on GM block learn, on Fords fuel trim forgot what Chrysler/ Jeep calls it. You do this by touching the sensor wire with one hand and touching the positive or negative battery terminal with the other hand, this gives it enough resistance to not hurt anything but drives the injection either lean or rich.
