[Canuck623]

02's do have a life span of sorts. After 150 or so K miles they can become coated and the readings will shift but if there is no light those readings are withing the allowable range. If I was going to change any I would change both pre cat sensors instead of the post cat ones.

[jfjohn]

Thanks. Pre cat ones already were changed because they kicked on the light.

[Canuck623]

If your mileage has slipped change them. Not a lot of money. If the mileage isn't that bad they are probably ok. If you can watch them on a scan tool they should be nice cross counts about every couple seconds from low to high and back.

[Skyhawk]

Do you have a scanner that can monitor the O2 sensors in "real time". If you do then look for the "switching", from rich to lean, in "real time". The switching should be rapid and continuous. If the switching is slow then it is a good bet that the sensor is failing or failed. If your sensor is original with that milage I would bet that it is failing. The usual life of an O2 sensor, or change interval, is 100,000 miles. Also note that the downstream O2 sensor does not kick in (usually) until the car is warmed up. So, when you do you "real time test" let the vehicle warm up to operating temperature. Ditto the above, that is a diagnostic "tree" is very helpful in determining if the sensor is bad, or if you have a poor connection (or no connection in your case), a grounded wire, shorted wire, broken wire, pin out, or some other problem, other than a bad sensor. On old high milage cars, especially in the rust belt, corroded connections are a common cause of codes and problems.