My 2005 altima has an alt light on. It starts up ok but does that mean I need a new alternator? What does it mean?

just buy this car like that, not told any history.?
never have I seen any IMMO security failure allow engine to run so my guess means doors ajar or the like.(only, body alarm stuff)
battery lamp means alternator is dead. charge lamp. it is wired directly too lamp to alternator pin.
it means ALT is bad, or its fuse to it blown
or battery cells shored, and ALT goes off line to prevent battery explosion. (ruptured and ACID all over things horror)

if cranking sounds normal the battery is ok for now , but will die soon.. 13-15vdc at the battery car idling
if lower volts ALT is bad, or 3 fuse blown to it.

The warning light means the alternator is not producing power to send to the battery. The light is grounded thru the alternator and once the alternator starts charging the voltage reverses and the light goes out.
You should test the alternator output on the truck and there may be a problem with the wiring.

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The light means the alternator is not charging the battery. Once the charging circuit is completed, the warning light looses its ground connection.
You may need to double check the circuit going from the alternator to the battery, or the warning light circuit going to the alternator as it could be shorted to ground.

very possible u have a faulty alternator. uneed to chek battery voltage at battery w an ohm meter and engine runnin, visually look at alt belt too see if its even there or mayb loosend up.possibility. u should see 13volts or better at idle an or a slight revvivng of engine at meter. also have ur fuses checked for a faulty fuse supplying pwr to alt. good luk hope this helps

try a new battery sounds like its died

put a volt meter on the battery at idle it should read approx 13.5 volts. if it does not the alt is not charging the battery. That does not mean for sure that its bad, connections and battery power to the alt wil need to be verified. Is the battery light on, if so, the alt isnt charging.

the starter sounds about right if the battery is ok is the car a standard if so make sure the clutch pedal is al the way in if auto make sure the car is in park

I am assuming that when you say no alt light on dash , you mean that when you first turn key on for the bulb check it won't light. i f this is true then I would be looking at replacing the bulb first. most car's use the bulb in the dash as part of the excitment circut to get the alt to start charging.

Always clean the battery terminals before jumping. An extremely dead battery may not accept a jump as it serves as a 'buffer" instead of helping. If you do get it started, charge battery with a battery charger then have alternator and battery tested for proper operation and check belt condition and tension as well.