act up

behave badly or not work properly (a machine, child, or body part)

B2

What does "act up" mean?

To say that a machine or body part is 'acting up' means it is behaving unreliably or causing problems, even though it has not stopped working completely. This is the key distinction: something that has broken down has stopped working, but something that is acting up is still functioning — just not as it should. You might use it when a car keeps making a strange noise, a printer jams at the worst possible moment, or an old knee injury starts causing pain again. The expression is informal and very common in everyday spoken English, particularly when talking about health niggles or annoying mechanical problems. It often appears with 'again', which captures that frustrating sense of a recurring issue.

Examples

How to use it

subject (body part / machine) + act up

This is the core pattern — the malfunctioning thing is always the subject, and there is no object.

The boiler has been acting up since the temperature dropped last week.

subject + have/has been + acting up

The present perfect continuous is the most natural tense because problems with machines and body parts tend to be ongoing or recurring.

My shoulder has been acting up ever since I started working from home.

subject + act up + again

Adding 'again' reinforces the idea that this is a repeated problem rather than a one-off fault.

The wifi is acting up again — I can't get a stable connection.

subject + act up + in/under + condition

You can add a phrase to explain the trigger or circumstances that cause the malfunction.

My old hip injury tends to act up in cold weather.

it + act up

When the subject is already clear from context, a pronoun is often used instead of repeating the noun.

Have you had a look at the engine? It's been acting up all morning.

Common Collocations

my back / knee / shoulderthe car / enginethe printer / computerthe boiler / heatingmy phonemy old injury

Common Mistakes

Trying to add an object

'Act up' is intransitive — the broken or misbehaving thing must be the subject, not the object. You cannot put anything between or after the verb and particle.

✗ My body is acting it up again.
✓ My body is acting up again.
Confusing 'act up' with 'break down'

'Act up' means something is causing intermittent or partial problems but is still working; 'break down' means it has stopped working entirely. Use 'act up' for unreliable behaviour, not complete failure.

✗ My car acted up in the middle of the motorway — it completely stopped and I had to call for help.
✓ My car broke down in the middle of the motorway — it completely stopped and I had to call for help.
Using a person as the subject for this sense

'Act up' with a human or animal subject means 'behave badly', which is a different sense entirely. For the meaning of malfunctioning, only machines, devices, and body parts work as subjects.

✗ The mechanic acted up and couldn't fix my car properly.
✓ The car acted up and the mechanic couldn't find the cause.

Usage

This is an informal, everyday expression used mainly in spoken English. 'Play up' is the common British English equivalent and means exactly the same thing.

Frequently Asked Questions

Does 'act up' always mean malfunction? I've heard it used about children.

No — 'act up' has two distinct senses. When the subject is a machine or body part, it means 'malfunction or work unreliably'. When the subject is a person, especially a child, it means 'behave badly'. The subject of the sentence tells you which sense is intended.

Is 'act up' British or American English?

It's used in both, but it's more common in American English. British speakers often say 'play up' instead, which means exactly the same thing in this sense. Both expressions are widely understood across varieties of English.

Can I say 'my car will act up tomorrow'?

This sounds a little unnatural, because malfunctions are usually described as ongoing or past experiences rather than predicted future events. It's more idiomatic to say something like 'my car might start acting up in the cold' or 'I'm worried the engine will start acting up'.

What kinds of things can 'act up' as subjects?

Typically body parts (back, knee, hip, stomach, heart) and machines or systems (car, engine, boiler, printer, phone, heating, wifi). You can also use broader expressions like 'the whole system is acting up' as long as the subject is non-human.

Can 'act up' be used in formal writing?

It's best avoided in formal or professional writing — it belongs to informal, everyday spoken English. In a technical report or formal email, you'd be better off using a phrase like 'functioning intermittently' or 'experiencing issues'.

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