Diagnosta wykrył usterkę.

Because some masculine personal nouns in Polish end in -a. This is normal for words like mężczyzna, kolega, kierowca, and diagnosta.

So even though diagnosta looks a bit like a feminine noun, it is grammatically masculine here. You can see that from the verb too: wykrył is the masculine singular past form.

Wykrył is:

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

It means that the action was completed: the diagnostician detected/found the fault.

This -ł ending is very common in masculine singular past tense verbs:

• zrobił = he did
• znalazł = he found
• wykrył = he detected

Because usterkę is in the accusative singular, not the nominative.

The noun usterka is the direct object of the verb wykrył, so Polish changes its form:

• nominative: usterka
• accusative: usterkę

This is very common with feminine nouns ending in -a:

• książka → książkę
• kobieta → kobietę
• usterka → usterkę

Polish does not have articles like English a/an/the.

So Diagnosta wykrył usterkę can mean things like:

• The diagnostician detected a fault
• The diagnostician detected the fault
• A diagnostician detected a fault

The exact meaning depends on context.

Yes, often there is a difference in nuance.

• wykryć = detect, discover, often something hidden, hard to notice, or revealed by inspection
• znaleźć = find

In this sentence, wykrył sounds very natural because a diagnostician typically detects a defect rather than simply finds it.

Yes. Polish word order is fairly flexible because endings show grammatical roles.

The most neutral order is:

• Diagnosta wykrył usterkę.

But you could also say:

• Usterkę wykrył diagnosta. — emphasis on who detected it
• Diagnosta usterkę wykrył. — possible, but more marked
• Wykrył usterkę diagnosta. — also possible in the right context, but less neutral

So the basic meaning stays similar, but the focus changes.

A rough guide:

• diagnosta ≈ dya-GNOS-ta
• wykrył ≈ VY-krɨw
• usterkę ≈ oo-STER-keng or oo-STER-ke

A few important points:

• w sounds like English v
• y is a Polish vowel with no exact English equivalent
• ł sounds like English w
• final ę is often only weakly nasal in normal speech

Also, Polish usually stresses the next-to-last syllable:

• diaGNOSta
• WYkrył
• usTERkę

Wykryć is a perfective verb.

That means it presents the action as completed: the defect was successfully detected.

Its imperfective partner is wykrywać:

• Diagnosta wykrywa usterkę. = The diagnostician is detecting / detects a fault.
• Diagnosta wykrył usterkę. = The diagnostician detected a fault.

For learners, the important idea is:

• perfective = completed whole action
• imperfective = ongoing, repeated, or general action

The verb would need a feminine past tense form:

• Diagnostka wykryła usterkę.

Compare:

• wykrył = masculine singular
• wykryła = feminine singular

So in Polish, past-tense verbs agree with the gender of the subject.

Then you usually get:

• Diagnosta nie wykrył usterki.

Notice the change:

• positive: wykrył usterkę → accusative
• negative: nie wykrył usterki → genitive

This is a very important Polish pattern: with many negated verbs, the direct object changes from accusative to genitive.

They are similar, but not identical.

• usterka usually means a fault, defect, or relatively limited technical problem
• awaria often suggests a more serious breakdown or failure

So usterka fits well if the issue is something a diagnostician identifies during inspection or testing, not necessarily a total breakdown.