Answer: The most common faults are in the ignition and fuel systems. First, you need to make sure you have good spark, correct fuel pressure. Also, the problem could be with a sensor: crankshaft position sensor, camshaft position sensor, throttle position sensor.
Sensors that fail to send information to the computer may cause the engine to run less efficiently, and they can sometimes be the cause of an engine that won't start. One common culprit for this problem is the crankshaft position sensor, which measures the position and speed of the crankshaft.
Issues Starting the Vehicle
The most common symptom associated with a bad or failing crankshaft position sensor is difficulty starting the vehicle. The crankshaft position sensor monitors the position and speed of the crankshaft and other parameters that play an important role when starting the engine.
1) Identify the Problem
Slow crank: The starter cranks, but it does not produce enough engine RPMs to successfully start the vehicle. Click, no crank: The solenoid clicks, but the starter fails to crank at all. No click, no crank: The solenoid isn't clicking, and the starter isn't cranking.
The O2 sensor will not cause the no start. The no start can be caused by the fuel pump or the ignition system. You should check the engine for spark and fuel pressure, when it is not turning on to help narrow the problem down. The ignition system may be the coil, module, or pick up in the distributor.
Engine won't crank. If the electrical contacts inside the switch have failed, turning the key to crank the engine might not result in any response at all. Engine starts and stalls.
If your starter relay has gone bad, the electrical signal will never make it from the battery to the starter motor. As a result, your engine won't turn over - no matter how many times you turn the key. A faulty relay often produces an audible clicking sound when you turn your car.
Spark plugs ignite the air-fuel mixture that's injected into the cylinders. For spark plugs that don't spark dependably, it will cause an engine misfire that robs you of performance. Cracked porcelain insulators and worn electrodes are the common issue. If more than one spark plug fails, your engine may not start.
If your vehicle won't start, it's usually caused by a dying or dead battery, loose or corroded connection cables, a bad alternator or an issue with the starter. It can be hard to determine if you're dealing with a battery or an alternator problem.
The Starter
The starter is a tiny but powerful electric motor that spins a small starter gear. When you turn the key, power is sent to the starter which engages its starter gear with the engine. The starter then "cranks," creating that unmistakable sound of a car engine starting up.
If your vehicle idles roughly, stalls frequently, has a drop in engine power, stumbles frequently, has reduced gas mileage, or accelerates slowly, these are all signs your camshaft position sensor could be failing.
The crankshaft position sensor is the most important of all engine management sensors, and the engine will absolutely not run without it.
CAUSES OF DEFECTIVE CRANKSHAFT SENSORS: CAUSE OF FAILURE
Internal short circuits. Breaks in wiring. Wiring short circuit. Mechanical damage to the encoder wheel.
INSTALLATION FAULTS AND WEAR ARE THE MOST COMMON CAUSES OF DAMAGE TO THE CRANKSHAFT AND CRANKSHAFT BEARINGS. Softening of the bearing journals due to previous bearing damage or improper modification work, e.g. excessive regrinding.
If you ignore the symptoms listed here and your camshaft position sensor fails, your vehicle will not start. As a camshaft position sensor weakens, so does the data it transmits to the ECM. Eventually the data signal becomes so weak the ECM switches off fuel and spark delivery, and your engine will not start.