Since your vehicle is a 1994 (older than 2000) it will need to undergo all three parts of the smog inspection... tailpipe test, visual inspection and functional inspection. Please visit our
How To Pass The Smog Check page for detailed information about the entire inspection process.
As far as the modifications... it should be noted, any modification to the emission system of the engine will cause your Chevy Impala to fail the visual portion of the smog test, as well as the tailpipe emissions inspection; should the modifications be causing an increase in HC, CO or NOx. These three harmful pollutants are measured during the tailpipe test.
Since your vehicle is an OBD I model, it's computer will not be connected to the smog machine during the inspection, but the smog technician will inspect the Malfunction Indicator lamp (service engine soon light) to ensure it is not illuminated while the engine is running. It is only OK for the light to be on while the ignition is in the ON position. Soon as the engine starts however, the light must turn off. If it does not, it too will cause an immediate smog failure... even if the rest of the inspection passes.
It would be best at this point to visit a smog check station, and before having the smog test administered, asking for a smog pre-test. A smog technician will be able to tell you if modifications are made to the engine which may cause it to fail the smog inspection.
