I have a 96 Tacoma 2.4L with 109K miles that is idling rough. I have changed plugs, wires, dist. cap, fuel filter and cleaned throttle body and fuel injectors. Nothing I have done has helped, someone told me the O2 sensor may be bad. Is there anyway to test the O2 sensor?

 

Some OBD readers (like the one in my signature or they type used with a laptop computer) will allow you to see the signal from the O2 sensor while it is running on the vehicle.....sadly that is really the only way to test them (ie hot and at operating conditions). If either of the O2 sensors were to go bad my bet would be the one in front of the converter (ie the one closest to the engine), so you could just play parts replacement and try a new one to see if it makes a difference.