Timothy O'Brien

I have a 2002 Audi A4 quattro 3.0 6 speed. I bought the car a few weeks ago and it had a slight sputter, a sort of brief hesitation when shifting between 1st and 3rd around 2000rpm's and below. The owner said it was an O2 sensor and it was throwing a code for it, and he had the sensor. So I changed it a couple days ago and the check engine light went out and it ran perfectly for about a day. Then back to the sputtering. But it's not coding anything. Any ideas?