N1,
Must say that TGT failures in my experience are not that common.
The RB21 has 17 thermocouples that all route through a ballast resisitor.
Serveral themocouples can be u/s (Open circuit) but not have a dramatic effect on readings.
I have know up to half a dozen u/s with no marked indication problem.
There is usually a few degrees drop in indicated TGT
Of course there is also indicator failure but generally these are obvious faults
Sticking or FSD.