koshoushindan ga owari shidai, shutchouhi to gijutsuryou o setsumeishimasu.

I will explain the call-out and labor charges as soon as the diagnosis ends.

Breakdown of koshoushindan ga owari shidai, shutchouhi to gijutsuryou o setsumeishimasu.

• を: direct object particle
• と: noun-connector particle
• が: subject particle
• 説明せつめいする: to explain

Questions & Answers about koshoushindan ga owari shidai, shutchouhi to gijutsuryou o setsumeishimasu.

Why is が used after 故障診断?

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.

Why is it 終わり次第 instead of 終わる次第 or 終わった次第?

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.