For the
Fire and Overheat System Test, with the OVHT DET switches in NORMAL:
Place the TEST switch to FAULT/INOP
If the Master Caution lights, OVHT/DET annunciator, FAULT light and APU DET INOP light illuminate, the Fault and APU Detector Inop circuits are normal.
If the FAULT light fails to illuminate, the fault monitoring system is inoperative.
In this case I think you are AOG - I would have to check the MEL to be sure, and I don't have access to one at the moment
When you place the TEST switch to OVHT/FIRE, it is the illumination of the Fault light that tells you you have a problem in one of the loops. You then have to go throught the procedure to determine
which loop that is, disable that loop and continue under the MEL for one loop inop.