Your Jeep Cherokee is not equipped with an EGR system which means NO reduction is controlled by proper fuel control and the Catalytic Converter. Since replacing the CAT did not help your Jeep Cherokee pass the smog test we recommend looking into the fuel control. You need to ensure that your Jeep is not running lean (low CO emissions will indicate a lean fuel mixture) and that there are no vacuum leaks.
Other problems which could cause your Jeep Cherokee to have high NO include; engine cooling problems, lean fuel condition due to vacuum leaks (broken and/or disconnected vacuum lines) and high mileage (increased cylinder compression).
Bad Engine Cooling - NOx is created when an engine's combustion chamber temperature reach over 2500F. For this reason you want to make sure your Cherokee's temperature gauge is always normal and that your cooling system is working properly. If your Jeep's engine cooling system is not working efficiently chances are there is an excessive amount of NO being created.
High Cylinder Compression - Carbon build-up within an engine's combustion chambers will cause increased engine temperature and high NOx. Carbon build-up normally develops in an engine's combustion chambers over some period of time. The solution to this problem is called an Engine DeCarbonizing. It will remove a good amount of carbon of your engine's piston heads and valves. This will increase combustion space, lower compression and lower NOx.
Lean Fuel Mixtures - Lean fuel mixtures cause high NOx also. A lean fuel mixture is when the engine receives less fuel then is necessary to obtain efficient combustion. Lean fuel mixtures are often due to vacuum leaks. These leaks will allow the suction of additional and un-metered air into the combustion mixture or exhaust (depending on where the vacuum leak is located) disturbing pre/post fuel combustion and increasing NOx emissions.
