Alright folks, I just bought my A4 last night. It was a bit of a gamble from a BMW dealership because it ran a little rough, but it was a good deal and I'm pretty decent at wrenching... after I'm sure of the problem.

Well, 15 miles into the 160 mile drive home and the check engine light came on.

The symptoms changed a bit after the test drive and on the way home.

In P or N, the car idles perfectly. Put it in gear and there is a shudder like the torque converter is trying to catch. In D, as soon as I let off the brake, the car moves forward like normal and the shudder dissappears.

Normal driving that doesn't require much throttle, the car is smooth. Go up a hill and put the car under a load and the shudder returns, at interstate speeds going uphill it gets even worse with a shudder/surge that seems to shake the whole car.

I spent the day seaching forums and just went to AutoZone to pull the codes.

I feared the dreaded Torque Converter Code, but I ended up with these codes:

P2138 and P0421

From what little info I could find on here and other forums, it looks to be something with the throttle and something with the catalytic converters.

Since I am really new to the car, could you guys tell me if those codes and the symptoms match up?

Thanks in advance!

- mike