The fault light QRH:
ENGINE FIRE/OVERHEAT
DETECTOR FAULT
Condition: Engine fire and overheat detection is
inoperative.
The fire detection system in
one or both engines is
inoperative.
As you say,
on the ground, you can check system A or B and select the operative loop or Normal if they are both good.
The APU is a single loop an thus warrants the MC as a fire could go undetected.. V bad
