Power plants won't malfunction unless actually taking damage at the time (or tickeled with a malfunction weapon effect).
A standard malfunction will knock you down to 40% output for five seconds, so it may be wise to make sure your thrusters and FSD are below this and everything else on a lesser priority so you don't knock yourself out of SC during a hot scoop or something.
Once the PP reaches zero integrity it will fall to 20% output for five seconds, then stabilize at 50%. Any damage past this that would trigger a malfunction will instead cause the ship to explode. A reboot/repair can help here, but you'll still want to repair everything else as much as practical, both before and after.
Edit: Correction...I'm pretty sure powerplants haven't been able to be damaged by heat for a very long time, so about the only way you could trigger a PP malfunction in SC, that I can think of, would be with caustic cargo or something.