With the major systems checked out, there are a number of other things you can check to see why your car won't start - if you have the patience.
* Loose Starter: Loose starter bolts will cause it to dance around and wiggle, failing to turn the engine over.
* Bad Injectors: A bad injector can throw the whole fuel system off and keep the engine from firing, especially when the engine is warm.
* Faulty Cold Start Valve: A failed cold start valve will keep your car from starting when the engine is cold. It can also malfunction and keep your car from starting when it's warm!
* Chipped Flywheel or Ring Gear: Your starter's gear connects with the gear teeth on your flywheel or ring gear (depending on transmission type). If one of these teeth becomes worn or chipped, the starter will spin. You'll hear loud screeches, scrapes, squeals and grinding if this is the case.
* Bad ECU or MAF If your engine's main computer or any part of the system's electronics go bad, your car won't start. Unfortunately you'll need to leave this type of diagnostic work to a qualified repair shop.