MacBook shuts itself off at startup
So my 2011 Macbook Pro shuts itself off on startup, always right when the Apple logo shows up with a status bar at the bottom of the screen. The status bar fills up a tiny bit and boom, it shuts off.
Yes, there was a very small liquid spill many weeks ago, and this was happening then. I replaced the keyboard and thermal paste under the heatsink, cleaned the logic board, and dried everything out throughly, in the naive hope that that might help this issue. The machine worked great for a few days, and now the shutting off problem happens every time. My hopes, dashed!
I should probably add that the battery status was listed as "replace now" after the teardown I did, but it happens whether it's plugged in or not.
How can I find out what's causing the problem, as a non-expert? Please help!!
UPDATE: Here is the list of things I've already tried, if that's helpful:
-Replaced keyboard (previous one was definitely damaged) and thermal paste under heatsink
-Cleaned logic board and checked for visible corrosion
-Tried starting up without both the battery and 1 RAM in at a time (nothing)
-FSCK said disk could not be verified
-Ran disk utility and repaired main drive but "Macintosh HD" was greyed out
I can't run AHT since it crashes before I can get it running.