Untestable (redundant) fault
Beginner
A pretend flaw that no test can ever catch, because it doesn’t change what the chip does.
Novice
A fault that no input pattern can both trigger and make visible at an output. Redundant logic, wires tied to a constant and blocked paths cause them.
Expert
Reports separate faults proven undetectable (tied, blocked, redundant) from ATPG-untestable ones (not testable under the current test constraints) and aborted ones (the search gave up). Only the proven ones are fair to exclude from coverage.