To act up: To misbehave (for people) or not to work properly (for machines)
Let’s take a look at the phrasal verb “to act up”.
We use “to act up” to describe the bad behavior of a person, usually it is referring to children or teenagers.
NOW, let’s look at an example of each…
The teacher warned the students if they acted up in class, they would have extra homework.
My car has been acting up lately, I think I will have to take it to the mechanic.
Hope this helps!