Car did not pass smog check

As far as I remember was about 3 years ago.

Pull a spark plug and see what it looks like. New plugs and upstream oxygen sensors may be all you need.

I'll check the plugs but I don't even know what the upstream oxygen sensor is, but I'll look around on the Internet. Thanks by the way.

Single exhaust has two oxygen sensors. One before the catalytic converter and then another behind. Like a river, upstream and downstream.