O2 Sensor Question

A couple of months ago I would get an intermittent code 21, so I decided it would be time to replace the 160k old oxygen sensor. No more occasional MIL light and picked up 3mpg to boot. Last week I get the infamous no start. I immediately replaced the crank position sensor (one long extension, one u-joint, one 3" extension and an 11MM socket. All done underneath. in case you find yourself trying to figure out what combination of tools makes the job easy.) So, I hit the key, still no start. Then I pulled the codes and discovered code 54 (camshaft position sensor) I have reasoned that it was time to replace the crank sensor anyway so I made another trip to the dealer to pick up a cam sensor. Now it starts and runs great, but code 21 has mysteriously re-appeared. Any ideas on what might cause an O2 fault on the new sensor. Connectors are clean and the harness appears to be in good shape. MPG is still good and still runs great.
1995 XJ 4.0 AW4

I have a similar Code 21 on the MIL light and have checked it with a code reader and verified that it is a brand new factory O2 sensor I installed.
My next step is to install the old one and see what happens.
I will reply back after I do this or find something out.

Well I spent most of Sunday tinkering on the Jeep.
I replaced the old upstream sensor, the first one in the exhaust down tube, with no change.
I still get the 21 MIL code and the code reader shows P0132. At one point I also got a P0135. This refered to Sensor Heater Circuit Malfunction, but I cleared that and it did not come back.
I will have to talk to my trusted mechanic and/or one at a dealer and see what they can tell me.

Okay, here is what I was told by my mechanic.
He plugged in his sophisticated scanner/code reader and found only the O2 sensor fault code and no other issues.
It showed 4.6 to 5 volts on the screen.
He then checked the output with his multi-meter and confirmed that reading.
He said that I have a faulty O2 sensor and to replace it.
I will go down that path tomorrow.
I will post with any further updates.

The Check Engine light hasn't come on for a few days now. I guess time will tell if the problem appears again.

Did you clear the old codes???? If not they might come back until it reads correct info for like 5 key starts.

On 2 O2 systems alway replace both at the same time.

hinkley

I have a simple code reader device and used that to clear the light, Innova 3100.
I also did some other work on the Jeep and had the battery unhooked for 8 hours or so.
I did do both sensors.
Thanks for the replies.