The Mazda Miata is a great little roadster but even it can break down on occasion. Thankfully the Mazda Miata comes with an error-code system that can be read by an on-board diagnostics reader, also known as an OBD reader. This tool can diagnose the problem and display a specific code for that issue. You can then look up the code and determine if you can solve the problem yourself or if you are better off taking the Miata to a mechanic.
Put the car is in park and pull the emergency brake up. If the car is drivable, then drive it to an area that is off the street first before placing it in park. Turn the car off and take the keys out of the ignition. Look under the steering column for the diagnostic port. It will be near the kick plate on the left side of the driver's side.
Take an OBD reader plug and push it by hand into the diagnostic port under the steering column. If your OBD reader requires the car to be on then turn the key in the ignition halfway to the electrical power position, but not all the way to crank the engine. The reader will display the codes one by one on most OBD readers. Use the arrows to scroll through. If you have a higher end unit with a larger LCD display then it will display all the codes on the screen.
Copy the codes down on a piece of paper or onto a laptop, making sure to not mistype. A single wrong letter can throw you off entirely. Compare the codes to the Mazda code database or from a Mazda code book. Write down the problems and compare them to the problems you are experiencing with the Miata. If they are similar, then the problem has been correctly troubleshooted. If not, then there may be electrical problems. If the diagnostic system is failing, then you need a professional mechanic to take a look at it, preferably a Mazda-certified one.
