El radiador ha dejado de funcionar y ahora hace frío en el dormitorio.

Literally, ha dejado de funcionar means has stopped functioning.

It is built like this:

• ha = has (from haber, the auxiliary verb)
• dejado = past participle of dejar
• de funcionar = to function / to work

So dejar de + infinitive is a very common pattern meaning to stop doing something.

Examples:

• Ha dejado de llover = It has stopped raining
• He dejado de fumar = I have stopped smoking

So here:

• El radiador ha dejado de funcionar = The radiator has stopped working

In Spain, the present perfect (ha dejado) is very commonly used for recent past events that still matter now.

That fits this sentence well:

• the radiator stopped working recently
• the result is still relevant now
• now it is cold in the bedroom

So in Spain, ha dejado de funcionar sounds very natural.

You could also hear:

• El radiador dejó de funcionar...

That is grammatically correct too, but in Peninsular Spanish it can sound a bit more like a finished past event, depending on context.

Very roughly:

• ha dejado = has stopped / has gone and stopped
• dejó = stopped

They are similar, but not exactly the same.

• No funciona = It doesn’t work / It isn’t working
• Ha dejado de funcionar = It has stopped working

The second one emphasizes a change:

• it used to work
• then it stopped

So:

• El radiador no funciona = the radiator doesn’t work
• El radiador ha dejado de funcionar = the radiator has stopped working

The sentence you gave sounds more specific and more natural if you want to highlight that the problem started.

In Spanish, weather and ambient conditions are often expressed with hacer.

So:

• hace frío = it is cold
• hace calor = it is hot

This is just the normal Spanish structure.

You would not usually say:

• es frío for it is cold in a room

That sounds wrong for this meaning.

Useful comparisons:

• Hace frío en el dormitorio = It’s cold in the bedroom
• El dormitorio es frío = The bedroom is cold by nature / generally a cold room

So hace frío describes the current temperature or feeling in the space.

Ahora means now, and it helps connect cause and result:

• The radiator has stopped working
• and now it is cold in the bedroom

It is not absolutely necessary, but it adds a clear sense of consequence and current situation.

Compare:

• El radiador ha dejado de funcionar y hace frío en el dormitorio
  = The radiator has stopped working and it’s cold in the bedroom

• El radiador ha dejado de funcionar y ahora hace frío en el dormitorio
  = The radiator has stopped working and now it’s cold in the bedroom

The version with ahora feels a bit more natural if you want to stress the result.

En means in, so:

• en el dormitorio = in the bedroom

That is the normal preposition here because you are saying where it is cold.

Examples:

• Hace calor en la cocina = It’s hot in the kitchen
• Hace frío en el baño = It’s cold in the bathroom

So:

• en = location
• el dormitorio = the bedroom

Together:

• en el dormitorio = in the bedroom

Spanish often uses the definite article more than English does.

So where English might say:

• in the bedroom

Spanish normally says:

• en el dormitorio

Leaving out the article here would usually sound incomplete or unnatural.

This is very common with rooms and places:

• en la cocina = in the kitchen
• en el salón = in the living room
• en el dormitorio = in the bedroom

Because Spanish only contracts:

• a + el → al
• de + el → del

But:

• en + el stays en el

So:

• Voy al dormitorio = I’m going to the bedroom
• Vengo del dormitorio = I’m coming from the bedroom
• Estoy en el dormitorio = I’m in the bedroom

That is why the sentence correctly says en el dormitorio.

In Spain:

• dormitorio = bedroom
• habitación = room, often also used for bedroom depending on context
• cuarto = room, but it can sound more informal or regional

In your sentence, dormitorio is very clear and specific: it means the room used for sleeping.

Examples:

• Hace frío en el dormitorio = It’s cold in the bedroom
• Hace frío en la habitación = It’s cold in the room / bedroom

If the meaning is specifically bedroom, dormitorio is a very good choice.

Yes, it sounds natural.

El radiador ha dejado de funcionar y ahora hace frío en el dormitorio.

Why it sounds natural:

• radiador is the normal word for radiator
• ha dejado de funcionar is a common way to say has stopped working
• ahora nicely links the result
• hace frío is the standard expression
• dormitorio is perfectly normal in Spain

A slightly simpler version would also be natural:

• El radiador no funciona y hace frío en el dormitorio.

But your original sentence sounds a bit more descriptive and expressive.

Yes, absolutely.

Funcionar is the normal verb for machines, devices, systems, and appliances working / functioning.

So these are all natural:

• El radiador funciona = The radiator works
• La calefacción no funciona = The heating doesn’t work
• El ascensor ha dejado de funcionar = The lift has stopped working

For a radiator, funcionar is completely standard.

Not usually for this meaning.

If you want to say it is cold in the bedroom, the normal expression is:

• Hace frío en el dormitorio

Está frío is possible in other contexts, but it usually refers to something being cold rather than the general ambient temperature.

For example:

• El radiador está frío = The radiator is cold
• La habitación está fría = The room is cold

So:

• Hace frío en el dormitorio = the usual way to talk about the temperature in the room
• El dormitorio está frío = possible, but it focuses more on the room itself being cold

In your sentence, hace frío is the most idiomatic choice.