Finally after nursing it to a local shop i got my answer and yes i'm pissed but happy it will be fixed...turns out you can't put two twelve inch subs in the car and run it on a stock alternator and small battery...lol The cel code it gave was p0685...ecm power supply relay. a 15 dollar part to fix my car that dies...wow.

 

the part just failed. but you can run the subs no problem. the battery size is good enough as long as you replace it every 3 years or so as recommended and unless your alternator is on its way out it should charge it just fine. If you havent replaced it acid batteries become a stress point on the alternator with time as it takes a lot more juice to fully charge them. If you keep your music on at max bass all the time then the amps could become a problem, but i think you'll be spending more money on advil than on the car if that were the case.

 

I am going to have both the battery and alternator tested but the battery looks newer and that battery has a 7 yr warranty and it doesn't seem stock...its a duralast...as far as the alternator that im not sure...it must charge the battery ok because the first time the car randomly died i didn't know what was going on and my buddy sat there and kept trying to start the car and we kept trying different things but the battery held up nicely. I would think constatly cranking the engine to try to sart it would kill the battery fairly quickly. The fact that after scanning the codes and the only one that came up was the ecm power relay that cost me 30 bucks ill start there and just pull my subs all together. the car has 87 thousand miles and just had the timing belt inspected by audi which took all day just to find out every component was fine. not sure but ive been through the typical troubleshooting and have already done a complete tune up at this point so why not. If it isn't the relay ill go from there.

 

I finally got it!!! The relay was bad but that was not the issue. I tested the alternator and battery and both checked out..I got frustrated and borrowed the obd 2 scanner from autozone and it finally threw a code i can use. Turns out it is in fact the crankshaft position sensor/rpm sensor. the actual code says poor connection not necessarily a bad sensor so tomorrow im going to pull it and clean it with electronics cleaner. If not ill replace it.

 

Hello I know this thread is kind of old but i am having very similar issues... I bought both the ecm relay and have replaced the engine speed sensor.. my question is.. where is the ecm relay located and how do I access it??? any help would be greatly appreciated.

 

yeah this is a little old but here was my solution...

I ended up doing a complete tune up...coilpacks, spark plugs, fuel filter, just had the oil changed along with some small things like checking fuel delivery, starter, alternator and battery and still nothing.
I also replaced the ecm power relay and fuel pump relay. Those are both located in a plastic housing just above the air box but thats on a amu 225. there is a plastic cover and under that should lye both relays. I wasnt sure which one it was and audi said my car shouldn't have one so i did both. But that wasnt the issue. You said you replaced the engine speed sensor? That was what solved the issue for me. its the sensor that has a grey plug on one end and a magnet on the other. the grey plug plugs in just to the right on the oil dipstick and the magnet bolts in next to the oil filter and is a pain to get to. I did manage to do it without removing the bumper which is recommended but there is little room to work. best thing there is to remove both air hoses but be careful as they break easily, one of mine did.

If that isn't the issue the next thing i was going to do was the hall sender or whatever they call it. They go often from what i am told. It is located right in the plastic timing cover on the top right as you are looking at the cylinder head from the passenger side.
The issue is that with that the car should run but run like crap. If the car is dying and wont start for awhile maybe you got a bad crank sensor/rpm sensor.

the last thing which would suck royaly is electrical from the speed sensor to wherever it goes.

Depending on what type of car it is...225 or 180 and the miles I would say to start cheap and work up...All of the things I replaced "could have" been the issue says audi and many shops around. Personally after what I went through though the car should run even if it is like crap if it anything other than that sensor.

Oh and timing belt...mine just broke and luckily I should be covered under that lawsuit settlement thing.