I've read from this forum that if the 02 sensor is the problem, then
I will have one of the dashboard lights like check engine (CE) light light up. But I've never had that problem. And I know the lightbulbs are working since when I turn the ignition key to start the engine, all of the dashboard lights up. I also read that the 02 sensor only kicks in once the engine has warmed up.
As I mentioned in my post, my car has a little rough coldstart for one second, then the engine becomes smooth.
But I've also never changed the 02 sensor since I got the car at 75K miles. It now has 115K miles. Could it be defective and but the CE light doesn't light up.