Trouble Code P0420 indicates a problem with your Nissan Maxima’s three-way catalytic converter or not as efficiently as the ECU is expecting. The problem can be due to the CAT not functioning properly or a defect with the rear oxygen sensor which is used to report the condition of the exhaust exiting the CAT. If the O2 sensor itself is defective the ECU may interpret the sensors results as a defective catalytic converter. It is best to insure both rear oxygen sensor and catalytic converter are working properly.
Regarding trouble code P0420… Nissan has put out a service bulletin (see link below). As far as assistance with repairing your Nissan’s smog check problem we recommend looking into the CAP Consumer Assistance repair program.