koukanbuhin ga todoki sidai, syuttyousyuuri wo simasu.

Here, 次第 means as soon as or once.

So 届き次第 means as soon as it arrives.

This pattern is common in formal or business Japanese:

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

Examples:

• 準備ができ次第、連絡します。
  As soon as the preparations are ready, I’ll contact you.
• 確認でき次第、お知らせします。
  I’ll let you know as soon as I can confirm it.

In your sentence, 交換部品が届き次第、出張修理をします。 sounds like a business/customer-service statement.

Because 次第 attaches to the ます-stem of a verb.

The verb is 届く
Its ます form is 届きます
The stem is 届き

So:

• 届き次第 = correct
• 届いた次第 = not this grammar pattern
• 届く次第 = also not used for this pattern

This is just how the grammar works:

• でき次第
• わかり次第
• 届き次第
• 終わり次第

It may feel a little formulaic, and that is normal. It is a set expression used especially in formal written or spoken Japanese.

交換部品 is a compound noun:

• 交換 = replacement, exchange
• 部品 = parts, components

Together, 交換部品 means replacement part(s).

A useful thing to know: Japanese nouns usually do not clearly mark singular vs. plural unless needed. So 交換部品 could mean:

• a replacement part
• replacement parts

You understand the number from context.

が marks 交換部品 as the thing that does the arriving.

In 交換部品が届く:

• 交換部品 = the replacement part(s)
• が = subject marker
• 届く = to arrive

So literally, it is something like:

• The replacement part(s) arrive
• or more naturally, when the replacement part(s) arrive

English often uses word order instead of a subject marker, but Japanese uses particles to show the role of each word.

Not exactly. 出張修理 is more specific than just repair.

• 出張 = going out on business, making a visit, visiting the customer’s location
• 修理 = repair

So 出張修理 means something like:

• on-site repair
• a repair visit
• repair service at your location

It suggests that the company or technician will come to the customer, rather than the customer bringing the item somewhere.

Yes. This is very common in Japanese.

Many nouns can combine with する to make a verb-like expression.
So:

• 修理する = to repair
• 出張修理をする = to do an on-site repair / to carry out a repair visit

The particle を marks 出張修理 as the thing being done.

This noun + をする pattern is extremely common:

• 勉強をする = to study
• 連絡をする = to contact
• 予約をする = to make a reservation

So 出張修理をします is a natural and standard way to say it.

Japanese often leaves out subjects when they are clear from context.

In this sentence, します implies that the speaker’s side will do it, such as:

• the company
• the repair staff
• we

So even though we is not written, the meaning is understood from the situation.

This is very normal in Japanese. Sentences often omit:

• I
• you
• we
• they

if the listener can figure them out from context.

Yes, it is fairly formal and sounds very natural in business or customer-service Japanese.

Reasons it sounds formal:

• 次第 is more formal than casual time expressions like たら
• します is polite
• the wording is compact and service-oriented

This is the kind of sentence you might see in:

• an email from customer support
• a service notice
• a message from a repair company
• a receptionist or technician speaking politely

A more casual version might use something like:

• 交換部品が届いたら、修理します。

That is still correct, but less formal.

Both can mean something like when / after / as soon as, but the nuance is different.

届き次第

• more formal
• often used in business or announcements
• emphasizes acting promptly once something happens

届いたら

• more everyday and conversational
• less formal
• simply means when it arrives

So:

• 交換部品が届き次第、出張修理をします。
  formal, business-like: As soon as the replacement parts arrive, we will perform the on-site repair.

• 交換部品が届いたら、出張修理をします。
  more neutral/everyday: When the replacement parts arrive, we’ll do the on-site repair.

In customer-facing communication, 次第 often sounds more polished.

Not always. 次第 has several uses in Japanese, which can confuse learners.

In this sentence, it is the grammar pattern:

• Vます-stem + 次第
• meaning as soon as ...

But 次第 can also appear in other expressions, such as:

• 〜次第です = it is because of..., it depends on...
• なり次第 = as soon as...
• 詳細は別紙のとおり次第 — actually this would not be the same pattern, and this kind of overlap is why learners get confused

So when you see 届き次第, think of it as one fixed grammar pattern, not just the dictionary meaning of 次第 by itself.

Yes, it can be omitted, especially in shorter writing.

So both are possible:

• 交換部品が届き次第、出張修理をします。
• 交換部品が届き次第出張修理をします。

The comma makes it easier to read by clearly separating:

• the timing clause: 交換部品が届き次第
• the main action: 出張修理をします

In formal writing, punctuation is often used to improve clarity, even when it is not strictly required.

Yes. 次第 often suggests that the second action will happen promptly after the first condition is met.

So this sentence does not just mean:

• after the parts arrive, we’ll repair it someday

It suggests:

• once the replacement parts arrive, we will proceed with the on-site repair without unnecessary delay

That is why it is common in service language. It sounds efficient and professional.

Yes, and that would sound very natural too.

For example:

• 交換部品が届き次第、修理に伺います。

This means:

• As soon as the replacement parts arrive, we will come to perform the repair.

Compared with 出張修理をします:

• 出張修理をします focuses on the service being performed
• 修理に伺います focuses on politely saying we will come to you

伺います is a very polite humble verb, so it is especially common in customer-service situations.

So the original sentence is natural, but there are other polite ways to say the same basic idea depending on what the speaker wants to emphasize.