First you'll need to get a diagnosis to find out what is causing your Nissan Maxima to produce high NO. The cost of the repair will be determined after the problem is found. It could be something as simple as a bad oxygen sensor which shouldn't cost more than $150.00 to replace, if it's only one sensor that's defective, to expensive as needed a new catalytic converter which can run into the hundreds. The first step is getting a diagnosis. A smog repair station will usually require 2 hours of diagnostic time to figure your Nissan Maxima's smog check failure. The cost for diagnosis will be calculated using their labor rate times the diagnosis hours needed or spent. Typical labor rates are $50.00 to $75.00 an hour.
Here's some information about NOx. NOx is created when an engine's combustion chamber temperatures exceed 2500F. This could be caused due to vacuum leaks, fuel control problems, engine overheating, and /or carbon build-up (high engine compression).
Assuming your Nissan Maxima did not fail HC or CO (since you don't mention it in your question) we should advise you this does not necessarily mean that your HC and CO levels are correct. CO (partially burned fuel) is needed in order to keep NO levels low. So we need to know if your Maxima is in proper fuel control. In order to control NO your car should be running 14.7/1 air/fuel ratio. Low CO or HC can cause high NOx. Having extremely low CO or HC is not necessarily a good sign. On a late model vehicle, the situation is a bit different. These vehicle's were designed to produce extremely low emission levels. 0 CO, 0 HC and 5-50 NO is not uncommon. The same does not apply to older vehicles.
The two main reasons vehicles fail for high NO is due to a defective EGR system, and second, is due to defective fuel control. After ensuring the EGR system is functioning you'll need to make sure adequate fuel is getting to the combustion chambers, and there are no vacuum leaks. Unmetered air is basically air entering the engine which is not being calculated by the emission computer, and the computer not adjusting fuel increase correctly. If any air (oxygen) enters the combustion chambers which is not measured by the Mass Air Flow sensor, your Nissan's engine control computer will not know to add fuel to the air, thus distrupting the air/fuel mixture (leaning it out) and causing high NO.
You'll need to have a smog check repair station ensure your Nissan is in proper fuel control. The diagnosis should begin with inspection of the fuel injection “feedback” system. This includes ensuring the oxygen sensors are working properly, the computer system is receiving the signals from the oxygen sensor, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery to the combustion chambers.
Along with the oxygen sensor several other emission sensors need to also be inspected during the fuel feedback test such as the mass air flow sensor (if equipped) and the manifold absolute pressure (MAP) sensor, engine coolant temperature sensor, and throttle position sensors. These sensors also play an important role in assisting the ECU with proper fuel control.
At the end of the feedback test it will be know whether your Nissan Maxima is failing for high NO because of an electronic problem or mechanical, and what needs to be done to fix the problem.
