Step 1: RC Cars
Another problem that may arise is, especially on lesser quality wheel and trigger remotes, is contact issues. Many of these types of controllers use metal contacts to connect to copper plates on the transmitter circuit board. Sometimes these contacts can become disconnected. The way to fix this is to unscrew the transmitter and take out the circuit board. Now, you can either bend the metal contacts toward you or screw the circuit board in closer by clipping off some of the screw pole and screwing the circuit board back in.
Also check for burnt-out parts in the circuit boards and loose components. If you have a burnt-out component, you can try to replace it, but it is generally very difficult. A better idea would be to call the company (if it's still under warranty) and try to get a replacement.
Moving away from electronics, sometimes a piece of plastic, especially in the steering system, can get broken and render the system useless. You can fix this generally by using a strong glue to secure the piece in place. In addition, screw pole can get cracked by the screw itself. What I do for this is squeeze the pole, glue the outside, and zip tie the pole together.
Cars are meant to be driven around, but that also encourages crashing. If you crash, it is important to diagnose what is wrong before you try to fix anything. I have many times fixed a problem on the outside of a car, glued it closed, and realized there was another problem inside. Make sure and take caution when fixing to not make any irreversible changes unless absolutely necessary.
Step 2: RC Boats
As far as electronics go, many of the same rules in Step 1 apply except for 1: electronics can get corroded if allowed contact with water. If this happens, treat it as a burned out component.
Other problems can occur with the driveshaft, especially the coupler that connects the driveshaft and the motor shaft. The driveshaft can get stripped, which usually requires a replacement, as do other stripped parts.
Mechanically, most problems in boats can be fixed by car solutions. If your problem isn't in the boat section, please refer to Step 1 and it may be there.
Boats can be tedious to maintain, but are very fun to use. Make sure you take care of your boat according to the directions.
Step 3: RC Planes
Step 4: Replacing the System
Sometimes, even though you've tried everything you can do to fix a vehicle, there is no fixing it. However, if it is an electronics issue, I often simply replace the entire remote control system with speed controllers, servos, batteries, and transmitters bought from Hobbyking. It is very inexpensive and often cheaper than buying a new vehicle. Remember, try anything and everything you can before attempting to replace the system.