1994 Jeep Cherokee Jeep Code 21

Are you using OEM parts? 02S shorted to voltage check the wiring all the way back to the computer

Yes I am using a Bosch PN15704 O2 sensor. What should the voltage reading be?
I have checked for a short with an ohmeter and there is no short. This is a heated O2 sensor can there be a problem there? I have checked the reesistance cold and it is 3 to 4 ohms and hot the resistance is 6 to 7 ohms is this correct? I am using a haynes manual and it gives very little info.

Check the voltage to it-I don't have the resistances specs-the voltage should be .45volts if its switching properly from lean to rich and vice versus-

OK so what should I do if the input viltage is too high. Is that a bad ECM? Is it another problem? I don't get a lean or rich condition.

You look for what's causing it to go lean such as below

A lean fuel condition can be caused by:

* Low fuel pressure due to a weak pump or leaky fuel pressure regulator. (use a fuel pressure gauge to check fuel pressure at idle)

* Dirty fuel injectors. (try cleaning the injectors)

* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body. (Use a vacuum gauge to check for low intake vacuum)

* Leaky EGR valve. (Check operation of EGR valve)

* Leaky PCV Valve or hose. (Check valve and hose connections)

* Dirty or defective Mass Airflow Sensor (MAF). (Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires)