With today's computer controlled cars, the possibility of a vehicle not starting when you turn the key is less likely then ever before. But it does happen, and when it does, it would help if you knew some basic tests and procedures that could allow you to determine the cause and often fix the problem yourself instead of relying on your local repair shop to bail you out.
The first step is to narrow down the cause of the no-start.
Let's go over the process of starting the car, so you have a better understanding of what is going on when you turn the key:
Here is what happens on a properly running car:
You sit behind the wheel and insert the ignition key into the switch.
You then turn the key to the spring loaded start position. When you do that, the ignition switch engages the starter by connecting the battery to the electric starter motor which, in turn cranks the engine over. This can be easily heard and is referred to as cranking the engine over.
The next thing you will hear is the engine running, which is your signal to release the key.
At that point, the engine is running and you are ready to place the transmission selector in Drive and be on your way.
An easy way to test for spark is with an inexpensive spark tester. This is a device that is readily available at most auto supply stores. You use it by simply holding it next to a spark plug wire. If you see the neon lamp flashing while someone cranks the engine, then you have spark and should move on to checking for fuel. If there is no spark, or a very weak spark, you will have to do a series of methodical tests that vary depending on the type of vehicle. You will need a repair manual for your car in order to get the correct diagnostic procedures. A good source for on-line repair information at a reasonable price is Alldata-DIY.
First step here is to listen for the fuel pump running inside the gas tank. When you turn the key to run, you should easily hear the pump come on, run for a few seconds to build fuel pressure, then turn off. If you do not hear it, it could mean that the fuel pump or circuitry is bad. (Fuel pump failure is a common problem on modern cars.)
Fuel injected cars are very sensitive to proper fuel pressure. If the pressure is off, even by a few pounds, it will cause noticeable performance problems, or a no-start condition. To check for proper fuel pressure, you will need a fuel pressure gauge that is suitable for your type of system. A fuel injected engine (found on just about every vehicle less than 20 years old) produces very high fuel pressures and requires a fuel pressure gauge that reads up to 100 pounds per square inch. This type of gauge has a threaded connector that must match the pressure tap on your fuel rail. Since you are working with a highly combustible fluid which can be quite dangerous if you do not know what you are doing, you should leave this step to a pro.
If you know that you have spark and fuel, the next step is to check for compression. For this, you will need a mechanic's grade compression tester that will screw into a spark plug hole. You will need to remove the spark plugs and use the compression tester to test the compression on each cylinder. If the compression is very low on all cylinders, that is a sure sign that the timing belt (or timing chain depending on the engine) has failed and will have to be replaced.