P171 after O2 sensor replaced

I just replace an O2 sensor due to error code P138 coming up on a scan. Now I'm getting a P171 (Lean) with rough idle but apparently running well at speed after a few short trips totaling about 60 miles. Is this one to worry about or does the computer just need more time to adjust to the new sensor?

The XJ in question is a late 1996 (OBD2) with 200K+ miles and was running great with no problems prior to P138 message.

The 2nd o2 sensor is there to monitor the performance of the converter, and plays no role in air/fuel mixture, so therefore shouldn't cause it to run lean.

It might just be a coincidence you're getting another code, but at 200K if the other o2 sensor (that is the one up front, that does control air/fuel mixture) is still the original, than it's long overdue to be replaced anyways.

Did you clear the codes, disconnect the battery for an hour to clear the memory of the the old sensor.

I did take it in and get the codes cleared but didn't think about disconnecting the battery to clear the memory. Will try leaving it disconnected while I'm at work today then drive it some this evening after I get home.