It won't run or it won't fly. What happened? Are the batteries dead? Is it out of fuel? Is there a short in the electrical system? Is there frequency interference? Learn how to troubleshoot operating problems with your electric and nitro RCs.
Get solutions to some common radio problems in this Feb 2003 article from Radio Control Boat Modeler.
If your helicopter isn't flying right, vibrates, has radio problems, or is jerky, try these troubleshooting tips from Heliguy.com.
From the Publishers of RC Car Action this book is a compilation of the magazine's "Troubleshooting" column and is full of fast fixes and problem-solving solutions.