while the car is running unhook the negative battery cable. If the car remains running then your alternater is good if it stalls then replace it.
Usually it means your alternator is bad or going bad.
Harsh start, lights dim when stereo is turned up with bass. Or go to advance auto and have them test it.
Bad horn - see if it is getting power when switch is activated Bad switch Bad ground
Check for fuel delivery (bad fuel pump?) Check for spark (bad coil?) Plugged fuel filter?
From my experience, If your crank sensors bad the car will not start up at all.
Bad timing belt, get replaced soon.
Turn car on remove positive battery terminal if car turns off alternator is bad
When your battery does not get charge.
Not a bad idea to replace both at the same time
Typically you know your alternator is out when you have to jump start your car numerous times. A way you can test it is by turning on your car and while it's running discount the + terminal. If the car turns off then your alternator is bad. If the car remains in then alternator is good and possible bad battery. Hope this helps
If it is makeing a noise or is not putting out at least 13.5 volts it is bad. Take the car or the alternator to an Auto Parts store and have them load test the alternator to be sure.
Bad/shorted battery? (doesn't not hold a charge)