Here are a few things the consider regarding your Jeep's smog check failure:
Your 1994 Jeep Cherokee is not equipped with an EGR valve, and thus the
reduction of NOx is the complete responsibility of the catalytic converter.
Assuming all other emissions system are working properly, the CAT must
reduce NOx emissions. First however we must know what the other emissions
numbers (CO and HC) look like. If both CO and HC are within average limits,
we can assume the CAT is defective, is not lowering NOx as needed and caused
your Jeep to fail the smog check. If either HC Hydrocarbon or CO Carbon
Monoxide is borderline failing, there may be emissions faults other then or
as well as the CAT.
Two other common issue which create high NOx besides the CAT are... Your
Jeep has a lean fuel mixture and is producing high NOx. This should be
indicated by extremely low CO results. The lean fuel mixture may be due to
vacuum leaks or defective fuel control (bad oxygen sensor, or other
emissions sensor). And the second common fault; Bad engine cooling.
Inefficient engine cooling will cause high NOx as well. Remember NOx
(Nitrous Oxides) is created when an engine's combustion chamber temperatures
exceed 2500F.
