Your Nissan Altima only has a front oxygen sensor. The majority of OBD-I equipped Nissan's (as opposed to the more complex OBD-II system) are only equipped with one oxygen sensor located in front of the Catalytic Converter.
Since you've cleaned your Nissan Altima's EGR valve we'll assume you've also ensured the EGR passages between the intake and exhaust manifolds are open and free form any debris, carbon deposits and/or obstruction. If this is the case, the next thing to verify is that in fact the EGR valve opens properly when vacuum is applied, and that vacuum is applied as necessary when your Nissan Altima is off idle and during cruising. If you manually apply vacuum to the EGR valve during idle, you should notice a RPM decrease. This shows the EGR valve is opening and allowing inert exhaust gas to enter the intake manifold.
Next you'll need to make sure that your Nissan Altima is in proper fuel control. In order to control NOx your Nissan Altima should be running 14.7/1 Air/Fuel mixture. The two leading faults which cause vehicles to produce high NO is a defective EGR system, and the second is defective fuel control.
You'll need to have a smog check repair station ensure your Nissan Altima in proper fuel control. The diagnosis should begin with inspection of the fuel injection “feedback” system. This includes ensuring the oxygen sensor is 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.
Your Nissan Altima's fuel feedback test should be conducted using a 5-gas analyzer (smog stations usually will use their smog machine as a 5-gas analyzer) and propane. Propane will be added to your Altima's intake manifold and the oxygen sensor will be measured for an increase in voltage. Using a voltmeter the smog technician should observe the voltage output of the oxygen sensor as propane is added. Voltage should increase and the ECU should in turn decrease fuel delivery through the fuel injectors.
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, 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 Altima is failing for high NO because of an electronic problem or mechanical.
