Symptoms of bad O2 sensors?

[Cerwin Vega Fan]

I'm trying to see if one of my O2 sensors is bad. My car has a OBD1 ECU so I'm not sure if it will throw an SES light if one of my O2's is bad. My car has been running real rich and blows black smoke out the exhaust (more than normal) every now and then under WOT conditions. Is their anyway to check for a bad O2 sensor?

Thanks

[Injuneer]

OBD-I has several codes that will set on failure of the O2 sensors. Unlike OBD-II, though, the OBD-I PCM does not looking a "switching rates", which can be a measure of gradual deterioration and contamination.

The engine will only run rich at part load/throttle conditions if the O2 sensors are provding faulty feedback, or if the root cause of the rich condition is so severe that the long term fuel corrections can not control it. However, should the latter occur (maxing out the long term corrections) you will get an SES light and a code.

Faulty info from the sensors, leading to a rich condition can be:

-faulty sensor element(s)
-faulty sensor heater(s)
-faulty wiring on the sensor or the heater
-exhaust leaks before the sensor
-high misfire rates

Often, a scan data log will help identify the cause of the problem.

[wrd1972]

My Z28 has 70K miles and i switched the O2's just for the heck of it. The left one was severely fouled and the right side looked more normal. Mine blew black spray out the tailpipes when you started it up but I also feel I had bad plugs as well. I replaced my crappy Bosch plats with NGK I's. I also cleaned the MAF and the throttle body.
Now my car is running better than ever, it idles smoother and doesnt have that jerky feel at light accelaration like it used.
I never got any codes pointing to anything piticular but Ive been told that bad O2's can screw things up.
The left sensor is easy to change and the right one is a PITA, just a little heads up.