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My 1987 Mazda RX-7 failed the smog test for high HC and NOx. Can it be a bad oxygen sensor?

I have a 1987 Mazda RX-7 na automatic. It did not pass smog. Hc and no2 were to high. Changed the plugs and wires and hc came down a little and no2 went up. What should I try next. o2 sensor? then the cat. Does this car have a pre-cat. Iam doing the work myself to save money and don't want to waste money or time fixing the wrong thing.
Answer:

The O2 sensor could be next. A certified smog check repair station will have to find out if your vehicle is in proper fuel control. The diagnosis should begin with inspection of the air/fuel "feedback" system. This includes ensuring the oxygen sensors are working properly, the emissions computer system is receiving signals from the oxygen sensor/s, 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. The feed back test will utilize a 5-gas analyzer (the smog machine can be used for this test) and propane. The smog technician will introduce propane to the intake system and ensure the oxygen sensor is sensing the propane as an increase in CO (carbon monoxide), and ordering the emissions computer to decrease fuel delivery the instant propane is added to air intake stream. This test should be done after the engine has sufficiently warmed up and while it is running at idle. If no change in fuel delivery is seen there is feedback problem. The diagnose then should turn to which part of the feedback system is not working right. Using a multimeter voltmeter the smog repair mechanic should observe the voltage output of the oxygen sensor as propane is added. The voltage coming from the oxygen sensor should increase. If no voltage increase is seen the O2 sensor is not working. The sensor should be replaced and the test started over. If voltage increase is noticed the smog technician needs to ensure voltage increase is also present at the computer input terminal. If voltage is present at the terminal as well next the technician must ensure the emissions computer output to the fuel injectors are lowering "injector pulse rates". At the end of the fuel system feedback test it will be know whether the high emissions your vehicle is producing is an electronic or mechanical fault. The technician can then further diagnose the failure. posted by SmogTips Support

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