Learn French: J'allais contacter le garagiste, mais il était occupé. - I was going to contact the mechanic, but he was busy.

By putting the verb aller in the imparfait (allais) and following it with an infinitive (contacter), you create the past equivalent of the near future tense. It translates to 'I was going to contact'. We use this specific structure to talk about things we intended to do in the past but did not end up doing.

What would the sentence mean if I used the passé composé and said Je suis allé contacter?

If you use the passé composé, it changes from a past intention to a completed physical action. Je suis allé contacter means 'I went to contact' (implying you physically traveled there and actually did it). The imparfait J'allais keeps it as a mere intention: 'I was going to'.

Why is the second verb, était, also in the imparfait?

The imparfait is used for background descriptions and states of being in the past. Because being busy (occupé) describes an ongoing state rather than a sudden, completed action, you need the imparfait form of être (il était).

Can I use le mécanicien instead of le garagiste here?

Yes, you can. Un mécanicien is a general mechanic who might work on planes, trains, or cars. Un garagiste specifically refers to an auto mechanic or the owner of a local car repair shop (un garage), making it the most natural choice for everyday car troubles.