And as a last resort
Light may be INOP providing proper system operation is verified prior to each departure.
-Start engine and let stabilize at idle.
-Engine bleed and APU bleed switches OFF
-Eng AI switch OFF
-Record idle EGT
-Switch eng AI on, let EGT stabilize and take note (30 seconds maximum)
-Switch eng AI off, take note of stable EGT
If EGT increased with minimum 15C and returned to recorded idle EGT afterwards system is verified as operational and dispatch with light INOP is authorised.
Procedure to be repeated prior to each departure.
(On the NG its easier because you have the TAI indication on the screens as well, then above is not needed)