The number one reason RadioShack gets Xmods returns and repairs are for busted servos. The servos only steer in one direction, and won't move past the center position to go the other direction. CoRnDoG lost 2 Supras this way, until I discovered the fix. My local RadioShack repair center recently confirmed that this is often the way they fix servos.

Remove your servo (along with the other electronics) from the chassis. You'll probably need to remove both the front and rear drive systems before you can do this.

Here's what the servo looks like, with the horn (the stick that moves the tie rod) removed:

There are four screws, two in front above the servo horn/gear, and two in the bottom of the motor-side. Remove the screws, and start pulling the front of the servo off slowly. When you can see the gears, you will notice that they are interlocked. Pull the front away just enough so the bottom gear clear the box, then you can remove the front by moving it downwards.

You pull the front half downwards when the servo looks like this:

The gear on the bottom, which is connected to the front of the servo box, slides back and forth, so don't worry if it moves or comes off the axle.

Now that the servo is apart, look at the gears on the inside of the servo. All of the servos I have worked on had a line through the gears, but one person I've talked to said they could not find it.

In this picture, the line is slightly off center, pointed upwards from the center gear:

We'll call this the balancing gear.

This gear adjusts where the center position is for the servo horn, which is why I call this process 'servo balancing.' Normally centered servos have the line pointed straight up. A collision with the front tires probably makes the gears skip teeth, which throws off the centering of this gear.

Check your car and make sure none of your wires and circuit boards are touching anything they shouldn't be (No short circuits!), then put some batteries in and turn your car and remote on. The gears in the servo behind the balancing gear (the one in the picture) will probably start spinning.

Grab hold of the balancing gear, and turn it slowly. One direction will make the gears behind it turn faster, the other will make them turn slower. Figure out which direction you need, and turn the gear until the gears behind it stop spinning completely. When the gears stop, you have what I call a balanced servo.

Take the front end of the servo, and pull the gear outwards towards the end of the shaft. Carefully put the front back on. You need to match the gears between the two parts of the servo, while making sure not to let the balancing gear move. If the balancing gear moves while you're putting the front back on, your servo will be unbalanced again. Once your servo is back together, hold it shut and test your steering with the remote. Hopefully it works. If not.. it may be time for a return. If your car is out of warranty, read step X. Now is a good time to turn your car off.

Now that you have your servo out, you might want to center the servo horn. After that, finish reassemling your car, then do a final test.

Servo still not working? If you are frustrated enough to try force to fix it, here's how. Finish reassembling your car (step 5). With your car off, hold the front wheels, steer them all the way in the direction they can move in, then quickly and forcefully steer them in the other direction. Something inside the servo will click hard. You will either be able to steer properly, or the servo will be stuck steering in the other direction. Repeat a few times, and if you get lucky you'll end up with a working servo again. I've done this three times, each time a success, but forcing things won't always wield positive results.

Run your car around a few laps. I've had a servo re-lock on me twice. Once, re-rebalancing worked, the second time, I got an exchange. I try hard to avoid my local RadioShack now, because they seem to hate me for doing so many exchanges and repairs. If your car works, drive more carefully from now on.