Well, I changed the Spark Plugs successfully, but the inevitable has happened: I'm getting TERRIBLE gas mileage, which from what I've been researching, it sounds like the O2 sensors.
But this was dramatic. Just 3 months ago, this car was giving me 30mpg on freeway, and about 24 city. Right now, I put about 13 gallons last week, and it looks as if I'm going to get about 270 miles out of this. So, 20 MPG city/highway. I'd say about 2/3 is city, though.
But still, this decrease was very sudden. Just before the Eng Light came on, the car was fine. And then, light comes on, and all of a sudden, car's like it's going to die.
So, I'm holding off on the Coolant System to take care of the O2 sensors, if that's what you guys think I should do to fix the gas mileage (and eng light) issue.