So that means, if you don't have a engine code, you have a good sensor? Or can they be almost "sluggish enough" to rob some milage and still not throw an error code? In other words, is the MIL indicator a RELIABLE way to troubleshoot 02 sensor problems?
I've been very confused by so many people claiming to replace your 02 sensor preventitively if it's "old". Of course definitions of old vary from 1 yr to 100K, or "whenever your gas milage starts getting really bad."
To get us back on track (and avoid jacking this thread) I checked my OEM shop manual for trouble codes and voltages:
Various MIL lamp 02 sensor trouble codes: P0030, P0036, P0130-P0161 (these include two for sensor showing "sluggish response"), P1195-P1197.
The manual goes on to say that the 02S (oxygen sensor) can fail in any or all of the following ways: slow response rate, reduced output voltage, dynamic shift, shorted or open circuit(s). On my XJ (1999 model), I have two 02 sensors (one before, and one after the cat). Your '97 should be the same, but I'm not sure.
The output voltage range of the 02S vary from 0 to 1 volt, and can be anywhere in between based on detected oxygen concentration. As the sensor starts to malfunction, it can take longer to detect changes. To detect a shift in the A/F mixture, the sensor must detect a change within a threshold value; a malfunctioning sensor could have difficulty changing beyond this value.
To me, the last statement seems to imply that the computer may have trouble detecting a bad sensor, but I really don't have a clue!
Some other tidbits: Operating temp for the 02S is 300-350 degrees F, and the sensor generates a voltage INVERSELY proportional to the amount of oxygen detected in the exhaust, and are VERY temperature sensitive. The 02S heater element must be tested to insure it's attaining proper temperature.
The downstream sensor copies the voltages of the upstream sensor, with the only difference being the time lag between the two [question: does this mean they are the SAME type sensor in different locations? or are the two different types (non interchangable) sensors?].
It can take 2 failures in a row to trip the MIL, and 3 good trips in a row to reset it.
After all that, I think I'll stick to just replacing my with the low milage ones I bought off of ebay, just because I have 120k+ on my original ones. It actually seems easier than testing them (considering my MIL isn't on), and it only cost me $35 shipped...
Hope this helps!