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I don't know for defective.
for these two words, the result may be the same.
But "défectueux" is for a not perfect thing, There is a defect somewhere.
and défaillant is for something with a breakdown.
for example, the titanic had "des rivets défectueux" these rivets had a lower quality.
when the motor is "défaillant" the car doesn't start. maybe the motor is broken, and maybe a piece in the motor is "défectueuse"