1991 Subaru Legacy was sitting for 2-3 weeks and would not start. I got the battery jumped and it started and I was just going to drive straight to the shop, but when I got to the end of the road the car shut off and would not start again. Also prior to it sitting for 2-3 weeks it would start when I left from my residence, but if I drove it somewhere else and tried to start it again then it would not start. 
I have the following details: $1,500 worth of work done on it in 2015 including O2 sensor, ECM, fuel pump replacement, and new battery. The battery should still be under warranty. The car has an after market digital CD/cassette player, so I m not sure if that draws more energy than the car can handle. Also until recently one of the doors would not close completely and a little red light would be on constantly to indicate that the door was not closed. I m not sure if this light could have slowly been draining the battery/alternator or if this is even an issue. 
My question is based on the vehicle starting but not staying started, is this likely a problem with the alternator? Do I need a new one or could the current one be repaired? Cost estimate? 
I will have to get it towed to the shop because obviously it can t be driven in its current state, but I would just like to have an idea of what may be wrong and what would be the cheapest route since I have already spent so much money on the vehicle.

Best to put a meter on the battery terminals and measure the voltage before you start it and then after.  When it's running you should have 13.6 volts or more.

 

If you don't have a meter, but you do have a compass put it on the battery, when you crank the car the N will point in one direction, when it's running it should point in the opposite direction tell you it charging.

 

If you need an ALT, the best and least expensive route is a self serve yard.  Pull a Subaru Alt for about $45.

Not sure if this is related to your issue but I noticed on my '96 if it sat too long and the battery went dead, if I charged it only enough to get the vehicle started, then I could sort of start to move, but if I pressed the brakes it would stall and not have enough oomph to restart. If the battery was good and charged it was fine. In a pinch I would just not press the brake at all and use the parking brake so as to keep it from stalling out (this was just in the field / driveway not on the road). I think the brake lamps take a fair bit of juice and if the battery isn't good and charged the alternator doesn't respond fast enough so the voltage drops too low and the car dies.

 

How much/often does yours get driven? If it's only driven infrequently and for short trips it may not be fully recharging the battery.

 

Otherwise it might be good to check the parasitic draw with an ammeter and see if it's too high, there could be a circuit or something else causing it to drain.

batteries don't like to be completely discharged - even 1 or 2 times is bad - multiple times will destroy one.

 

maybe borrow a good battery from a working car, test voltages as mentioned too.

 

 

of course there are other possibilities, some older soobs had crank sensors that would fail when they warmed up, etc.

Edited February 3, 201610 yr by 1 Lucky Texan

Nobody has mentioned that most parts stores will check your battery and alternator for you, free of charge.  If you can get it there. 

Battery is dead. 

 

If you don't have a meter, but you do have a compass put it on the battery, when you crank the car the N will point in one direction, when it's running it should point in the opposite direction tell you it charging.

 

That is a handy trick, I could use my smartphone to check peoples alternator lol I'm going to have to try this.

might not be a good idea to put your smartphone near some megaOersted pole-flipping magnetic field.

 

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Use your wife's phone.

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Use your wife's phone.

 

 

Like that  idea. lol

 

O.