Weird backlight issues after liquid damage

I’m working on a 2017 MacBook Air that had some liquid damage on the power board (the separate board that has magsafe, USB, and aux).

At first it would not power on at all but I could see immediately there was some sticky residue around the solder points of the MagSafe connection and just cleaning that up got the system running but with no backlight.

I started investigating the usual suspects; backlight fuse, driver etc. and everything seemed to be OK and there was only some minor liquid residue with a little bit of corrosion on the under side of the board but it all cleaned up easily. Now the really weird thing is I can actually get the backlight working but it’s intermittent at best unless I boot it up with the battery unplugged.

I do this by holding the power button for 10 seconds then attaching the MagSafe and continue holding power button for 10 seconds then letting go. After that the backlight works consistently while in this mode even if I plug in the battery and restart it keeps working until I do an SMC reset.

The issue in this mode is the fan is always at 100% and the battery is not detected but it does run without the charger attached. If I run a system diagnostic in this mode I get error PFM001 (SMC error) and PPT001 (no battery) but if I run it in normal mode (when the backlight doesn’t work) there are no issues found.

Could the battery be causing this issue? There is a battery service alert while when running in normal mode. The fact that the backlight works without any issue in the other mode seems to show there are no problems related to any LCD components. Could it be the SMC? Is this other mode I’m referring to just a mode where the system bypasses the SMC?

The only other issue with the system is the speakers are disabled in the OS. They do work because I hear the chime sometimes when booting but usually not, I thinks it’s only once after doing an SMC reset.

Any help would be great, I’ve never seen this before and I’m very confused :S Thanks!