Check engine light is on.

My 95 jeep cherokee is lighting up the CEL. We just replaced the old motor with a different motor out of a 94 and now im getting code, 12,21, and 51. I know the cause of 12 and It will just take some time for that to go away but 21 and 51 are both related to the oxygen sensor. The sensor is the original one that was in my 95 with the old engine and it did not throw any codes.

21 - Neither rich nor lean condition detected from the oxygen sensor input.
or
Oxygen sensor input voltage maintained above the normal operating range.

51- Oxygen sensor signal input indicates lean air/fuel ratio condition during engine operation.

12- Direct battery input to PCM was disconnected within the last 50 key-on cycles.

What would cause my o2 to run above the normal operating range ? Why all of a sudden is it running lean ? Any help would be great!

is the wiring the same and is the ecu from the 95

the plugs connected right together, and the ECU is the original in the 95.

maby some where the wiring is different is the 94 and 95 o2 sensor identical. there wiring may be off. and if its a heating. o2 sensor you may be picking up the heating element causing the high voltage.

90xj06 said:

maby some where the wiring is different is the 94 and 95 o2 sensor identical. there wiring may be off. and if its a heating. o2 sensor you may be picking up the heating element causing the high voltage.

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Alright, ill take the sensor off the 94 and try that tommorrow.

well what im tring to say is compare them. but try it any way.cant hurt.

yeah, i think that would be a good why to check it with the O2 sensors from the 94

they are identical. The connector, the wire colors, the sensor. Everything is the same.

unless it needs a bit of time to re-learn after having the battery disconnected and having made the changes in engine?

its a possability tho i doubt it. maby wile changing the motor. you cought a wire and shorted it.
did the 95 you got the motor from have a working check engine light? and it may just be a bad sensor. could you imagin all this for that.

btw the sensor is obviously working if it is picking up a lean condition

yes it could or be freeking out because it is bad. but we will find out.