My "Test Only" smog mechanic ran my Jeep through a limited dyno run (at no charge) just to see if the jeep would pass this time (I had previously failed 4 times with him because of high NOx).

After changing just the one upstream o2 sensor late this morning, the NOx levels dropped from a staggering 1640 down to 640 in the 25mph test (would pass) and from 1450 to 720 in the 15mph test (would fail).

Now, he suggested that since I had just put in the new o2, the numbers might go even lower because the Jeep's computer (fuel/air mix management system) needs to re-learn how to read the new o2 sensor and how to manage the fuel mixture rates. He said that it could take about 75-100 miles.

I remember having to do this with my old 1997 Ford Taurus SHO when I changed the air intake system including AIT sensors, MAF, and a cat-back system.

My question is does the 1993 Jeep Grand Cherokee 4.0L Laredo (ZJ) really have that type of system that requires a "re-learning period" because of the o2 change?

Yup. Drive it around town and on the highway and give it a chance to reeducate itself.

Yes.

The system monitors and constantly adjusts itself as you drive. So if you have had a failing O2 sensor for some time and the system has adjusted itself to try and compensate. It will take time for the system to take the new information into account and adjust the system to compensate.

Take it for a good drive. Make sure to vary speeds and driving conditions to help the process along.

Thank you guys. I took my Jeep back into the smog shop today after having driven it hard for three and a half days after the o2 sensor change. It almost fully passed; the 15 MPH low speed test failed again. I'm thinking that one of the injectors is partially blocked/fouled. I bought a repair manual, but wanted to know if inspecting the injectors is something I can do on my own or am I better off having a mechanic inspect them?

Pull them, take them to an injector shop and let them go through them.

I thought there was a way to fully reset the computer's memory, too.

I know with my Mazda, you disconnect the battery and either turn on the headlights or pump the brake pedal (or both) to get any residual juice out of the electrical system, then reconnect the battery and drive as normal to let the computer begin rebuilding its memory. Aren't Jeeps similar?

You can pull both battery cables and touch them. This will discharge the memory-caps in the controllers and clear the adaptations.

It will search for idle speed when you first start it, but it will be fine in a few seconds.

SJ