Slow system and fan running all the time, despite resetting SMC.

I'm in need of some help with my girlfriend's MacBook Air. I have searched extensively but nobody seems to have all of the symptoms.

As soon as the system starts, the fan start to work at 100% speed, everything is slowed down (likely the CPU is being throttled). But, when I open activity monitor CPU is around 90-95% idle. Same goes for RAM and Disk usage. Thermal sensors indicate 38-40 Celsius and the system is not even warm to the touch.

I have tried almost everything, did SMC reset 5-6 times, booted in Safe Mode but the problem remains. Furthermore, MagSafe charger cord does not light up at all, but battery is charging.

As an additional information, system suffered from liquid damage around a year ago. Since we couldn't afford it we got it fixed via a 3rd party service provider (not the best idea but there was no other option) and the logic board was not changed.

It has been working for a whole year but now it suddenly started to show these symptoms.

Anything I could do by myself?

Should I just install a fan control utility to reduce fan usage since there is no overheat at all?

Then again I'm not sure how to get rid of slowness. Any hope for me?