Diagnosta skontrolował emisję spalin i podwozie.

Because it is the direct object of the verb skontrolował (checked/inspected), so it has to be in the accusative case.

• dictionary form: emisja
• accusative singular: emisję

This is a very common pattern for feminine nouns ending in -a:

• kontrola → kontrolę
• usterka → usterkę
• emisja → emisję

So in this sentence, skontrolował emisję means he checked the emission / emissions.

Because spalin depends on emisję and means of exhaust gases/fumes. In Polish, this relationship is usually expressed with the genitive case.

So:

• spaliny = exhaust fumes / exhaust gases
• emisja spalin = emission of exhaust fumes

In the sentence, emisję spalin is a whole noun phrase meaning exhaust emissions.

A useful way to think about it is:

• emisja czego? = emission of what?
• answer: spalin

Also, spaliny is normally used in the plural in Polish.

It is in the accusative too, but for this noun the nominative and accusative singular look the same.

• nominative: podwozie
• accusative: podwozie

That is very common for many neuter nouns in Polish.

So both objects are accusative here:

• emisję
• podwozie

Even though only emisję visibly changes, podwozie is also functioning as a direct object.

Skontrolował is:

• past tense
• 3rd person singular
• masculine
• from the verb skontrolować

So it means he checked / he inspected.

The -ł is a typical marker of the past tense, and the form shows agreement with a masculine singular subject:

• diagnosta skontrolował = the diagnostician/inspector checked
• if the subject were feminine, you would expect skontrolowała

For example:

• Diagnostka skontrolowała emisję spalin.

Yes. Skontrolować is a perfective verb.

That means the sentence presents the action as completed: the inspection/check was carried out and finished.

So:

• skontrolował = he checked / inspected fully, completed the check

Compare with an imperfective verb:

• kontrolował = he was checking / he checked (with focus on the process, duration, or repeated action)

In this sentence, the perfective form fits well because it sounds like a completed inspection report: he checked the emissions and the undercarriage.

Here diagnosta means a vehicle inspector / diagnostician, especially someone checking a car’s technical condition.

It is a masculine noun in this sentence and is the subject:

• Diagnosta = the inspector/diagnostician
• skontrolował = checked

Grammatically, it is in the nominative singular.

Yes. The verb skontrolował has two coordinated direct objects joined by i (and):

• emisję spalin
• podwozie

So the structure is:

• Diagnosta — subject
• skontrolował — verb
• emisję spalin i podwozie — two objects

In English, this is exactly like saying The inspector checked the emissions and the undercarriage.

Not by itself. I simply means and and joins two elements of the same kind.

Here it joins two noun phrases that are both objects of the same verb, so both stay in the case required by the verb: accusative.

So:

• skontrolował emisję
• skontrolował podwozie

combined:

• skontrolował emisję spalin i podwozie

No, Polish word order is more flexible than English word order because case endings show grammatical roles.

The given order is the most neutral and natural:

• Diagnosta skontrolował emisję spalin i podwozie.

But other orders are possible for emphasis, for example:

• Emisję spalin i podwozie skontrolował diagnosta.

That would put more focus on what was checked.

So the original sentence is standard subject–verb–object, but Polish can rearrange it more freely than English.

Because Polish does not use articles like English a/an and the.

So diagnosta can mean:

• a diagnostician
• the diagnostician

and the exact meaning depends on context.

The same is true for:

• podwozie = an/the undercarriage
• emisję spalin = the exhaust emissions or just exhaust emissions, depending on context

Both are correct, but in natural English it is usually best understood as exhaust emissions.

Literally, the Polish structure is:

• emisja = emission
• spalin = of exhaust fumes/gases

So word-for-word it is emission of exhaust fumes, but idiomatically exhaust emissions is the most natural translation.

This is a very common Polish pattern:

• noun + genitive noun
  instead of
• adjective + noun

So Polish often says something closer to emission of exhaust fumes, where English prefers exhaust emissions.