1994 obd code 21 51.

need some help fiquring out these codes on my 94.
using the key cycle method and couting flashes, i end up with 12, 21, 51.
12. battery disconectoed last 50 key cycles,
21 o2 sensor fault.
51 o2 sensor detects lean condition.

light has been on since a header repair,( replaced cracked manifold). and water pump.

sensor was disconected to remove the wiring harness from the intake.

testing the pins on the chassis side of the conector and with sensor disconectoed. i have .46 volts on the sensor + side, and continuity with the pcm to ground and heated ground.
what am i missing.

o2 sensor is 2.5ish years old. and good quality,

vehicle ran fine before repairs.
nothing i did should have affected the sensor.
94 4.0 aw4. 4x4.

thanks chris

code 12 you can ignore. and it looks like the o2 sensor may have died. did you remove the exhaust manifold and did the sensor get bumped?

code 21 is on because of code 51 being the o2 sensor is not switching to rich or it is infact reading lean.

did you check the o2 heater circuit when the jeep was running? not sure if it gets power when its not running.

If it's in fact reading a lean condition, check for an exhaust manifold leak, brake booster hose loose, an intake manifold vacuum hose loose, etc. Any one of those could have been 'bumped' during the work, and now creating a lean condition.

I put an o2 on it. Codes cleared instanlly. My guess is the sensor got some coolant on it from the lower radiator hose when it was removed.
Thanks for the help.