Look at the dash to see if the "Check engine" light is on. If it is, there is something wrong with the computer's electronic engine management systems. Get a code scanner from an auto parts store and pull the codes to find out what is malfunctioning in the engine management control. The computer stores any malfunctions in its memory and when interrogated will respond with a code, which is a series of numbers. These numbers can be translated into a diagnosis with the code sheet that comes with the code scanner.
If there were no codes and the car will not start, check the battery with a voltmeter. There should be 12.5 volts minimum. If the car will start with a jump and the battery is low, check across the battery again (with the engine running) for 14.5 volts--indicating that the alternator is working and the battery is the only problem. If the voltage is 12 volts, the alternator is bad and the engine is running off the battery and the battery is most likely fine.
If the battery has the right voltage and will not start, check the terminals. Listen for the starter to click when the key is turned to start. If it clicks, check the battery by checking the voltage when the key is turned to start by a helper. If the voltage drops below 10.5, the battery has the right volts but has a bad cell and no amperage. Replace it. If there is no clicking at the starter, check for battery voltage at the main wire on the starter. If there is no voltage, it has a bad cable. If the battery voltage is there, pull the small wire off the starter solenoid and check it for power when the helper turns the key to start. If it has power, the starter is bad; if there is no power, the ignition switch or security system is bad.
If the engine runs but is rough, check the ignition system by using a pair of insulated pliers and pull one plug wire at a time from the distributor. Or, disconnect the coils one at a time, listening for engine RPM change. If a cylinder is found that does not make a difference, this is the problem cylinder. Check the wires, if any, by spraying with water and looking for arcing on bad wires. Check the plugs, distributor cap and rotor, or the coil in coil-on plug applications. The easy way to check coil-on plug applications when one is believed to be bad is to swap one with another and test again. If that cylinder is now working, the coil is bad and needs replacing.
If the ignition is good, pull the connector off the injectors one at a time, looking for engine RPM drop. If any of the cylinders do not respond, the cylinder that does not respond has a bad injector.
If the engine has many miles on it and runs fair but smokes or makes noises, take off the oil cap and feel for air being forced out of the valve cover. This is blow-by, which means that a lot of combustion is passing the rings and entering the crankcase. If the oil pressure is very low, the bearings could be severely worn. An external oil pressure gauge should be used to confirm the oil pressure. It gets installed in the hole for the oil pressure sender. If it shows no oil pressure, look into the valve cover and see if oil is getting to the valve train. If there is, it could just be a sender that is bad. If there is no oil to the valves, the engine should be shut down immediately. Check the oil level and quality of the oil. If it is never changed, it could be so caked up inside the motor that the oil will not circulate any longer.
If there are noises in the bottom end of the motor that sound very hard, or solid knocking from start-up that continues while the engine is running, the main bearings are gone. If the knocking is not real hard but can be heard only when accelerating and goes away when decelerating, the rod bearings are bad. If there is an uneven rattling noise with the engine running, shut off the engine and check the water pump. Grab the fan blade and try to shake the fan and watch the water pump for a loose shaft. If it's loose, the pump is bad.
