If when you turn the key the engine turns over normally but will not start, there can be any number of reasons for this problem.
Some basic checks will need to be done to diagnose this problem.
1. Do you have fuel pressure.
Some cause of low or no fuel pressure are a plugged fuel filter, faulty pump relay, blown fuse, bad fuel pump. Different makes and models require different fuel pressures to start. Refer to a manual for the proper pressure for your application.
2. Is there sufficient spark at all the spark plugs.
No spark can be caused by many things. A faulty sensor, computer, ignition module, etc. Spark on some spark plugs can be caused by faulty ignition coil, spark plug wires or the plugs themselves.
3. Do the injectors fire and spray fuel into the engine.
No "Fuel Injector Pulse" can be caused by a faulty computer, wiring, or a computer. Pulse can be tested with a special test light. If the test light blinks, then the injector should fire. If not, then a plugged or failed injector is the cause. There are several special tool / tests to check the injector system.
4. Is the engine mechanically capable of starting.
Checking the internal components of the engine is required. Check for compression, see if the internal parts such as the Camshaft are turning, etc. There are also many tools to do these tests.
Question: Car cranks but wont start or stay running. Changed sparks plugs,coils ,crankshaft position sensor, camshaft senor.
Answer: You need to tell us what make and model vehicle you are asking about. This could be many different things wrong with your vehicle, and some are make and model specific. In general, you need to know if you are getting spark and fuel. Just changing all these parts does not help since we don't what the engine is missing.
If you have spark, then you need to find out if you have fuel pressure and fuel injector pulse signal. If no fuel pressure, there could be a bad fuel pump. If no fuel injector signal, this could be a bad PCM. Is the theft or security light on? If so then that is where your problem is.