My SES light came on and I brought it to the mechanic to scan it. The code came up P0131 low voltage O2 sensor. He drove it around and said at driving speeds the O2 was working fine but at idle the O2 seemed lazy. So I went and bought an AC Delco O2 sensor and put it in. I unplugged the battery for 15 minutes and started it and it idles funny. It will continue to go up and down slightly, you can watch the tach go up and down a little and you can hear it. Then when it sits and idles for about 3 minutes it will drop from about 800 and go down to 500 and then come back up and stays at 800 on the tach and the SES will come back on. The mechanic said there didn't seem to be any vaccum leaks and the fuel pressure is at 32 lbs. Could it be fuel injectors, plugs, wires,........?

Sounds like the same thing mine is doing. It could be your trim tables were reset, and that will take a few restarts and some run time to relearn. If your SES light is back on, get a scan and see what shows up.

So did you ever solve your problem? Ive been driving it for a few days with the new O2 and I just changed my oil today and when I started it, it was still idleing funny still.

I have not found my problem yet. I'm ordering a new O2 sensor and will install it next week. Other then rough idle, mines not giving me any problems. Mine started shortly after I installed my new PEMs Still hopping it'* just my trim tables doing a relearn.

i have the same problem, and no new parts installed:/
02 sensor voltage too low after replacing it twice :?
any updates?

I found my problem, it was a cracked EGR tube. Replaced that and problems went away. Not sure about the other guy.

If all you did was change the 02 and it is hunting for idle you may need to unplug the batt. for 30 min or more then allow it to relearn.
Other causes could be water in the fuel, dirty IAC, vac leak etc.

I was just reading up in my Funk & Wagnel and it states that the low voltage is referencing what the O2 is actually sensing and is not corrected after a period of xx minutes. Says it is running lean.
Here are some easy things to look for:

Hope that helps.

Ya I still haven't found the problem on mine. Where is the EGR on my 94, it is worth looking at and replacing I'm sure its been awhile? There are no vaccum leaks that I had seen, where is that purge canister connection at?

Purge canister is under the washer reservoir. One line goes to the fuel tank to catch vapors and the other goes to the vacuum "tree."
EGR: If 94 is same as 93 ... See the heat fabric covered pipe on the TB end of the engine (between TB and exhaust)?. That pipe is part of the EGR.