My check engine light is on and I get code 17 from the code block. I just replaced the O2 sensor with an aftermarket one and this made the light go off temporarily, but now it's back on again with the same code 17. How do I test the O2 sensor to see if it's functioning as it should?

I also saw on some other threads that idle mixture may be a factor. How do I test it and adjust it if necessary?

I don't know if it's related, but since I bought it two years ago the car has always had a rough idle when cold, along with a fluttering noise which seems to come from the smog pump(?) and goes away after about 30 seconds as the engine warms up.