It is absolutely necessary to have a smog check repair station conduct a hands-on diagnosis to find out what is causing your Jeep Cherokee to produce high NO. NO is produced when engine combustion temperatures rise above 2500f. Anything that will cause the engine to run hotter than normal will cause high NO; inadequate engine cooling and/or lean air/fuel ratio.
During your last smog check, according to your statements, the smog technician was able to reduce engine temperature by applying the Cherokee's air condition heater. The heater removed heat from the engine and applied it to the cabin; reducing NO. This was obviously not the fix your Jeep Cherokee needed, nor did it help solve future inspection faults.
You are now in a position that will require you have your Jeep Cherokee's high NO fault inspected and repaired. We recommend two things in particular.
First a smog repair station should conduct a fuel feedback test to ensure your Jeep Cherokee's engine control computer (ECC) is commanding the fuel injectors to increase fuel delivery when an increase in MAP (manifold absolute pressure) frequency is detected. The ECC, using MAP values, mathematically determines air volume. Assuming the MAP is functioning properly and the correct signals are reaching the ECC the ECC should command the fuel injectors to deliver more fuel to the combustion chambers when more air is sensed in the intake.
The same applies to the oxygen sensor only in reverse; with lower O2 voltage the ECC should command more fuel delivery and visa-versa. You mention having new oxygen sensors. Assuming the new oxygen sensors are working properly, a smog check repair station should make sure the new O2 are connected properly and their signals are reaching the ECC.
Mechanically we'd recommend inspecting the fuel injectors and fuel pressure. A sticking "closed" fuel injector will not allow enough fuel to enter the combustion chamber, causing a lean fuel condition and increasing NO. Your Jeep Cherokee will produce high NO when it is running lean or when the combustion chamber temperatures rise above 2500F. Things such as high engine compression and vacuum leaks should be diagnosed as well.
A smog station will charge between $50 to $85 an hour to perform a smog check diagnosis. Once the faults are found, additional funds will be required to perform the necessary repairs. If you think you may be eligible for low income repair assistance please consider looking into the Consumer Assistance Repair program to help fix your Jeep Cherokee's smog check failure.
