The weak functions are those in the tertiary, inferior, vulnerable, and role positions. Weak functions, in contrast to strong functions, exhibit:

Insecurity. The individual is hesitant or embarrassed about attempting to use these functions. They prefer for information, judgments, or actions related to these functions to be provided to them from external sources. For example, NJs prefer for others to supply Se, such as giving them a motivation or reward to do something difficult, rather than having to motivate or encourage themselves. TPs prefer others to supply Fe - to keep up a positive atmosphere, for instance - rather than having to check on others' moods themselves. Note that the tertiary function is often overconfident in its abilities, so although we prefer others to "supply" this function for us, we are often not consciously aware of this desire and will often mistakenly self-report a greater deal of confidence than we actually possess when the time comes to use it.

Hollowness. Judgments and interpretations using weak functions are often black-or-white, one-sided, or incomplete. They are liable to miss (or misconstrue) important information or events. This can often lead our strong functions to make errors that they wouldn't make if they were provided with correct information. For instance, an ENFP may misapply their energy in attempting to find solutions (using strong Ne) to a problem they don't completely understand (due to weak Te). If the Te is externally sourced (e.g., an ESTJ boss explains the problem to them), their Ne is likely to be much more focused and useful.

Fatigue. All weak functions induce fatigue, exhaustion, and frustration in the individual if they are required or requested to be used longer than the individual is capable of. The tertiary function is the 'least weak', followed by the role function, then the inferior, and finally the vulnerable function, which can fatigue the individual if they even think about using it. Note that the ignoring function, although a strong function, can also induce fatigue in the individual, although this is due more to boredom and frustration rather than weakness per se. Imagine the fatigue induced by working out a weak muscle (weak functions) vs. the fatigue induced by doing something easy but tedious and perceived as useless, such as blinking rapidly for five minutes straight (ignoring function).

Rigidity. Weak functions are limited in the ways that they can grow and incorporate new information. That said, it is important to note that they do learn and grow. By focusing one's effort on the appropriate methods - that is, methods related to the ways these functions actually grow - an individual can avoid excessive fatigue and frustration and produce meaningful results. Weak functions are either 2d or 1d functions.