kosyousindan ga owarimasita.

It is read こしょうしんだん.

• 故障 = malfunction, breakdown, fault
• 診断 = diagnosis

Together, 故障診断 means something like fault diagnosis, diagnostic check, or troubleshooting, depending on context.

That’s right—normal Japanese is usually written without spaces.

So in natural writing, this would normally appear as:

故障診断が終わりました。

The spaces are just there as a learning aid to make the parts easier to see:

• 故障診断 = the diagnostic process / troubleshooting
• が = subject marker
• 終わりました = finished / ended

が marks the subject of the sentence.

Here, the thing that ended is 故障診断. So the structure is basically:

• 故障診断が = the troubleshooting / diagnostic process
• 終わりました = ended / finished

So が tells you what it is that has finished.

Because 終わる is an intransitive verb.

An intransitive verb does not take a direct object. It describes something ending rather than someone ending something.

So:

• 故障診断が終わりました。 = The troubleshooting ended / is finished.

If you want to say that someone finished the troubleshooting, you would usually use the transitive verb 終える:

• 故障診断を終えました。 = We finished the troubleshooting.

That is one of the most important grammar points in this sentence.

終わりました is the polite past form of 終わる, which means to end or to finish.

So 終わりました can mean:

• ended
• finished
• has finished
• is finished

The best English wording depends on context. In many real situations, a natural translation would be The diagnostic check is complete or Troubleshooting has finished.

In Japanese, the past form is commonly used for a completed action or event.

So 終わりました does not only mean something happened in the past in a distant sense. It often means:

• the action is now complete
• the state has changed
• the result is now true

That is why in English it is often translated with has finished or is finished, not only simple past finished.

Literally, it is closer to The troubleshooting finished or The diagnostic process has ended.

Japanese often leaves out the doer if it is obvious from context. So in a real situation, this sentence may imply:

• We have finished troubleshooting
• The troubleshooting is complete

But grammatically, the sentence focuses on the process being over, not on who did it.

Yes, you could.

The difference is mainly nuance:

• 故障診断が終わりました。
  This presents the fact that the troubleshooting has finished.

• 故障診断は終わりました。
  This makes 故障診断 the topic: As for the troubleshooting, it’s finished.

Using は can sound more contrastive depending on context, such as implying that something else is not finished yet.

So が is a very natural choice when simply announcing completion.

Yes, here 故障診断 works as a compound noun.

Japanese often combines nouns directly:

• 故障 = fault, malfunction
• 診断 = diagnosis

Together they form one idea: fault diagnosis / troubleshooting.

No particle is needed between them because this is not a full phrase like diagnosis of a malfunction. It is a single compound term.

Yes. 終わりました is in the polite ます-form, so the sentence is polite and suitable for normal workplace or customer-facing situations.

Compare:

• 終わった。 = casual
• 終わりました。 = polite
• 終了しました。 = a bit more formal / official-sounding

So 故障診断が終わりました。 sounds like a normal polite statement such as you might hear in a workplace, repair shop, or technical setting.