Does anybody know how to tell if an O2 sensor is bad. I have an F150 that I am going to be working on this weekend. Its a 98 with 145000 miles. I have a ODBII scanner and can read the outputs of the sensors but I have no idea how to tell they are bad. Some people believe that they should just be changed but I hate to buy 4 new sensors without knowing which is bad. All the sensors are outputing values all the time.

The work is on a CEL code for insuffiecient EGR flow. I have researched that part and am pretty sure its just clogged with exhaust soot.

Any ideas how to figure out a bad sensor?

jtdwab