kosyousindan ga owari sidai, syuttyouhi to gizyuturyou wo setumeisimasu.

Here 故障診断 is treated as the thing that finishes, so it takes が as the subject of 終わり.

In other words, the sentence is structured like:

故障診断が終わり次第
= once the troubleshooting/diagnosis finishes

A natural English speaker might expect something more like after finishing the diagnosis, but in Japanese it is very common to say that the event or process itself finishes.

次第 in this pattern connects to the stem of a verb, not the dictionary form or past form.

So:

• 終わる → dictionary form
• 終わり → verb stem
• 終わり次第 → as soon as it finishes / once it is finished

This is a set grammar pattern:

• V-stem + 次第
• meaning: as soon as ..., once ...

Examples:

• 着き次第、連絡します。 = I’ll contact you as soon as I arrive.
• 準備ができ次第、始めます。 = We’ll start as soon as the preparations are ready.

So 終わり次第 is grammatically correct because 次第 follows the verb stem.

In this sentence, 次第 means as soon as or once.

So:

• 故障診断が終わり次第
  = as soon as the fault diagnosis is finished

This usage is fairly formal and is common in business, customer service, announcements, and written Japanese.

Be careful: 次第 has other meanings too, such as depending on or circumstances, but in V-stem + 次第 it specifically means as soon as.

They are similar in meaning, but not identical in tone.

• 終わったら = when/after it finishes
  More neutral and common in everyday speech.
• 終わり次第 = as soon as it finishes
  More formal, and it often implies the next action will happen promptly.

So in a service or business context, 終わり次第、説明します sounds more professional than 終わったら、説明します.

Because 出張費 and 技術料 are joined by と, forming one combined object.

So:

出張費と技術料を説明します
= I will explain the travel fee and the technical fee

Japanese often uses one particle after the whole list rather than repeating it after each item.

You could think of it as:

• [出張費と技術料] を 説明します

Here と means and.

So:

• 出張費と技術料 = the travel fee and the technical fee/service charge

This is the standard way to connect nouns in Japanese.

出張費 literally means a business-trip fee, but in service contexts it usually refers to the fee charged for coming out to the customer’s location.

So if a technician visits your home or office, 出張費 is often the call-out fee, on-site visit fee, or travel charge.

技術料 means the technical fee, labor charge, or service fee for the technician’s work or expertise.

So in this sentence:

• 出張費 = fee for coming to the site
• 技術料 = fee for the technical work itself

It literally means will explain, but in this context it often means something like:

• I will explain the charges
• I will go over the fees with you
• I will inform you about the travel fee and technical fee

So it is not just a dictionary-style explanation. In a real-world service situation, it probably means the speaker will explain what the charges are, how they apply, or how much they will be.

Japanese often leaves out subjects and objects when they are understood from context.

So this sentence does not explicitly say:

• who will finish the diagnosis
• who will explain the fees
• who is being spoken to

But from the situation, we naturally understand something like:

• We / I will explain the fees
• after the diagnosis is complete
• to you

That omission is very normal in Japanese.

Yes, it sounds fairly formal and appropriate for customer service or business communication.

Reasons it sounds formal:

• 次第 is a formal grammar pattern
• 説明します is polite -ます form
• the vocabulary is business/service-oriented

A more casual version might sound more like:

故障診断が終わったら、出張費と技術料を説明します。

That means almost the same thing, but it is less formal in tone.