Q: What are the most common reasons my car won't start?

Be sure any engine immobilizer system (security system) is not activated, thus preventing the car from starting. "New" batteries are not necessarily fully charged when you buy them over the counter. Consequently, it has to be confirmed that the battery is indeed fully charged. If you have a fully charged battery and yet when you turn the key the starter motor does not engage and turn the engine over, that means that either the starter motor/solenoid assembly is faulty or the electrical circuit supplying the motor has a fault. You have indicated though that the starter motor is new so you’ll want to test the starter circuit that begins at the battery and includes grounds, wiring (some of which is very heavy cabling), fuses, relays, the ignition switch and terminations. Basically, with the key held in the "start" position, the starter is either getting power or it isn’t. With the key in the "start" position, if the starter motor is getting power but it doesn’t work, the starter is condemned and replaced. On the other hand, with the key in the "start" position, if there is NO power to the starter, then the circuit is traced until the fault in the circuit is found. In addition to a faulty ignition switch, faults can include high resistances due to corrosion, wire strand breaks, or loose terminals, which can only be diagnosed using a voltage drop test. If you desire that a certified mechanic resolve this, please simply request a no start diagnostic and the responding certified mechanic will get the problem diagnosed and repaired for you promptly. Please let us know if you have further concerns or questions as we are always here to help you.