So I got this car about six months ago (no maintenance info; no way to contact the original owner), and since the cold weather has started, I've been having acceleration issues.
I am not cold-starting the car. I wait at least fifteen minutes for the engine to warm up. I've cold-started it once or twice, and it did not seem to make a difference.
The problems are only initial. Usually, once I've gotten above 45 mph for a few minutes, the car's golden. If I start while going down a hill, or on a flat surface, It's good. If I brake to a stop, it will have a problem picking up speed—but only if I do it within the first mile or so of my drive. If I brake after a few miles, the car speeds right away from it.
Usually, it's not bad. It'll pick up speed if I keep on the gas, and then be good after it hits about 50 mph. Last night, however, I was starting up a long hill, and I was losing speed. I went from 20 mph to 6 mph, at 4x RPMs.
Transmission fluid is burnt and nasty. I have a filter, and we're going to change the fluid today. Fingers crossed it doesn't clog it.
The code reader says that the car has a p0420 code, and the check engine light, vsc light, and trac light are all on or flashing (as is the flat tire light?) I've read that this is most likely to be the upstream o2 sensor, but I would prefer not to buy a $50 part, and then find out I need a new catalytic converter instead. So has anyone experienced anything like this?
I am not mechanically literate, so format any answers like you're writing a "for dummies" book, please (My father'll be helping/supervising. He's pretty decent with cars, but he's not a mechanic).
Please help and thank you very much for any help you can provide.
EDT: Having now checked the transmission fluid properly (as in, according to the pontiac manual, w/gear-shifting and warm engine rather than cold) the transmission fluid is low. You can laugh; it was pretty stupid. I'll see if adding more fixes the problem.
Updates pending, because some other idiot might find himself in the same straits.