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1994 Pontiac Firebird failed smog test due to high HC and NO levels

Recently just bought a used 1994 Pontiac Firebird, 3.4L 6 Cyl, had to take it for a smog check for registration. Took it in once and it failed due to high HC and No levels. Put new O2 sensors and though the HC went down to passing levels the NO levels went even higher resulting in another failed test and I was just wondering what that could possibly mean- a bad CAT? Also from the test results summary at 15 mph-RPM=1798, %CO2= 14.6, %02=0.8. At 25mph RPM=2000, %CO2=14.7, %O2=0.6 are these numbers unacceptable as well?

Answer:

You want to first look into the EGR valve (Exhaust Gas Recirculation Valve). Make sure the valve opens and the EGR passages are clear. The EGR valve opens when your Pontiac is cruising in order to lower NO levels (by cooling the combustion chamber). Once verifying the EGR system is working, you can move on to suspecting the Catalytic Converter. As far as your Firebird's O2 and CO2 results... the CO2 looks good; above 14% shows good chemical reaction. The O2 results are a bit high however. You want to see O2 maximum of 0.5. Check the EGR for proper operation. Next check the fuel feedback system to ensure the new oxygen senors you installed are communicating with your Pontiac's ECU (engine control unit), and then conduct a Catalytic Converter test. To conduct these tests several industry specific tools will be required. It is best left up to a smog technician to perform in order to ensure accurate test results. posted by SmogTips Support

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