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While individual cars have unique issues, all cars are prone to suffering the following: Dead battery, bad alternator, bad starter, bad thermostat, bad fuel pump, etc.

Here's how you can diagnose some of these problems to determine just why your vehicle is doing what it's doing. There are ALWAYS exceptions, but this will at least get you started on figuring out what the problem is.

Dead Battery
Usually it's pretty obvious if your battery is dead - you turn the key, and nothing happens. Your dash doesn't light up, the radio doesn't turn on, and most importantly, the starter doesn't spin.

The battery doesn't always COMPLETELY drain when it's bad, so you might get just enough power to slowly crank your motor for a few seconds, then it stops and you get a clicking from the starter.

The first thing you should know is that if your engine cranks like normal, but won't fire, it's probably not the battery that's the problem. I see a lot of people whose first reaction to their car not starting is to jump it - which doesn't help if the battery isn't the problem.


Alternator
Now, the battery had to die for a reason, right? You ask the most out of your battery when starting the engine - once it's running, the alternator takes most of the work onto its shoulders, simultaneously charging and sustaining charge in the battery.

In older cars, it was possible to run the engine on the alternator alone - you could remove the battery after start up and it would run, because there was very little electricity needed to keep it running. Modern vehicles have computers, electric fuel pumps, electric cooling fans and sensors and stuff that also draw electricity, so it's not likely that your alternator will be able to do all of the work alone, and will need the battery as a buffer zone.

So, if your battery is dead, and you jump it, most of the time the alternator will be able to sustain enough of a charge to keep the engine running. You might notice dimming lights and such as a result of having no charge in the battery, but it should run, at least for a little while.

If the alternator is bad, it will not charge your battery at all, so you might get an initial bit of power from the jump start, but the engine will soon die.

If your car runs fine at night, then is completely dead in the morning, it's more than likely your battery that has failed.

If your car is running fine, then dies out while you're driving it, and there's no battery power at all, it's most likely your alternator.


Starter
Usually when Starters fail, they just don't do anything at all. There's no click, there's no nothing when you turn the key. The lights all light up like normal, the gauge shows that the battery is good, but still, nothing happens when you turn the key.

This isn't ALWAYS the case, but every time it's happened to me, these were the symptoms.

Usually the solenoid (the smaller cylinder on the starter) is not engaging. A few good whacks with a stick might free it up so you can start the engine, but this isn't guaranteed.

Starters can also fail by way of the pinion not engaging the flywheel. This will result in a "whirrrrrrrrrrrr" type noise, instead of the usual cranking of the engine. Banging on the starter might temporarily fix this as well, but not all of the time.

There's also the possibility of your ignition switch failing, which means that turning the key is not activating the starter like it should. You can usually bypass this by jumping the terminals on the solenoid, but you need to know for sure which ones are which before attempting this.

If the vehicle is a manual, it would be easier to push-start it - turn the key to on (to provide electricity to the spark plugs, fuel pump, etc) and get the vehicle rolling. Put it in first gear (some use second gear) and once rolling, dump the clutch, and immediately push the clutch back in. This should roll the engine over, and if you disengage the clutch fast enough, the engine will continue to run on it's own. Always have someone help you and be ready to stomp the brake pedal if something goes wrong.


Fuel Pump
So the lights come on, the engine cranks and sounds normal, but the engine never fires. A lot of people use the term "Turn Over", which I think confuses some people - "turning over" doesn't necessarily just mean that the engine turns when you turn the key, "turning over" is usually meant as "The engine cranked, and then fired up". The act of firing up and running on it's own is "turning over".

If your engine cranks, but does not "turn over", or run on it's own, you may have a fuel delivery problem.

This can be due to lack of fuel (you ran out of gas), a failed fuel pump, or worse, more complicated problems with the fuel lines, fuel filter, or even computer problems.

The best way to test if the fuel pump is to get a fuel rail pressure tester, hook it up, and have someone crank the engine. If the pressure shows normal, the fuel pump is good. If not, well, you can start assuming there's a fuel delivery problem.

You might be able to free up the fuel pump (which can get gunked up and clogged) by pounding on the fuel tank.


Ignition (Spark)
If your car exhibits the same symptoms as above, it could also just as easily be that there is not spark to ignite the fuel. This can get a bit trickier to determine.

If your engine cranks but does not turn over, and your fuel pressure checks out/you have ruled out the fuel pump by some other means, then you might not have any spark.

To test this, you can try shooting a few squirts of Starting Fluid into the air intake to see if the engine turns over. Be careful as this will dry out the cylinder walls, so it's usually best to crank the engine without spraying a few times in between attempts to get the cylinders lubed up again.

The other, more straight forward way is to simply stick a screw driver into one of the spark plug boots, then place the metal of the screw driver about a quarter inch away from some engine metal - have someone turn the key and see if there's a spark.

If there is, it might be too weak to fire the cylinder - which means you'll have to do a voltage test.

Causes for no-spark could be failed ignition modules, corroded cap and rotor, failed ignition coils, or by some incredibly unlucky coincidences, all plugs and wires have failed at the same time.



Note that there are ALWAYS exceptions to every rule. What I've written here are just some things to try - it's not a comprehensive list of every possible way to diagnose your no-start situation, and the symptoms I described may not apply to every possible situation - they're just the most common ones that I have experienced.