El nivel de combustible está bajo.

Because estar is normally used for a current state or condition, and a fuel level is something that can change.

• El nivel de combustible está bajo = the fuel level is low right now
• Ser would sound unnatural here because it suggests a more permanent or defining characteristic

A helpful shortcut:

• ser = what something is
• estar = how something is

For dashboard-style messages, Spanish usually uses estar:

• La batería está baja
• La presión está baja
• El tanque está vacío

The accent mark is important because está and esta are different words.

• está = he/she/it is or you are from estar
• esta = this (feminine)

So in your sentence, está is the verb:

• El nivel de combustible está bajo

Without the accent, it would be the wrong word.

Because bajo agrees with nivel, which is a masculine singular noun.

• el nivel → masculine singular
• so the adjective must also be masculine singular: bajo

Compare:

• El nivel está bajo
• La presión está baja

Even though combustible comes after de, it does not control the adjective. The main noun is nivel.

Spanish usually connects nouns with de where English often uses a noun directly before another noun.

English:

• fuel level

Spanish:

• nivel de combustible

This pattern is very common:

• tank lid → tapa del tanque
• water pressure → presión del agua
• engine temperature → temperatura del motor

So de often works like English of.

Because combustible here is being used in a general sense, not referring to a specific fuel.

• el nivel de combustible = the fuel level / level of fuel

Spanish often leaves out the article after de when speaking generally or when the second noun functions like a category.

Compare:

• un vaso de agua = a glass of water
• nivel de aceite = oil level
• precio de gasolina = price of gasoline

You could include an article in some contexts, but it would usually change the meaning to something more specific:

• el nivel del combustible = the level of the fuel, maybe a particular fuel already being discussed

Here, combustible means fuel in a general sense.

It does not necessarily mean only gasoline. It could refer to:

• gasoline/petrol
• diesel
• fuel in general

In Latin America, combustible is a very common general word, especially in formal or technical contexts.

If you want to be more specific, you might say:

• gasolina = gasoline/petrol
• diésel = diesel

So:

• El nivel de combustible está bajo = general, neutral
• El nivel de gasolina está bajo = specifically gasoline level is low

In Spanish, descriptive adjectives often come after the noun, especially in straightforward, factual statements.

So:

• el nivel ... está bajo

This is the normal order.

With estar + adjective, the adjective almost always comes after the verb:

• La puerta está abierta
• La batería está cargada
• El nivel está bajo

So this word order is completely standard.

It sounds natural, but it is a bit neutral/formal and very common in:

• dashboard warnings
• manuals
• technical language
• customer service or repair contexts

In everyday speech, people might also say:

• Tienes poca gasolina = You have little gas
• Ya casi no tienes combustible = You’re almost out of fuel
• Está bajo el nivel de gasolina = less common, but understandable
• Te queda poca gasolina = You have little gas left

So your sentence is very natural, especially for a vehicle display or system warning.

Usually, no—not with the same meaning.

El combustible está bajo sounds odd because fuel itself is not normally described as low in this context. What is low is the level or the amount.

Better options:

• El nivel de combustible está bajo
• Hay poco combustible
• Queda poco combustible

English allows Fuel is low more easily, especially in dashboard language, but Spanish often prefers naming the level or saying there is little fuel.

Not in this sentence.

Here, bajo means low in the sense of level/quantity. Because the subject is el nivel de combustible, the meaning is clearly about fuel amount.

Spanish bajo can mean different things depending on context:

• low level: La presión está baja
• short/low in height: Es bajo
• quiet/soft volume: Habla bajo

So the meaning depends on what is being described.

Because the subject of the sentence is not combustible by itself. The subject is el nivel, which is singular.

So the verb is singular:

• El nivel ... está

If the subject were plural, the verb would change:

• Los niveles de combustible están bajos

Even though combustible is an uncountable or mass-type noun here, that does not affect the verb. The verb agrees with nivel.

A few common conversational alternatives are:

• Tienes poca gasolina. = You have little gas.
• Te queda poco combustible. = You have little fuel left.
• Ya casi no tienes gasolina. = You almost don’t have any gas left.
• El tanque ya va vacío. = The tank is getting empty.

These are often more natural in everyday speech, while El nivel de combustible está bajo sounds like a warning message, manual, or formal statement.

They are related, but they are used differently.

• bajo describes a level
• poco describes a small amount

So:

• El nivel de combustible está bajo = The fuel level is low.
• Hay poco combustible = There is little fuel.
• Te queda poca gasolina = You have little gas left.

In English, these can feel very similar, but Spanish chooses different structures depending on whether you are talking about the level or the amount.