Before anyone picks up a tool or books a shop, write a short statement of the job. It names the unit, the symptom in your own words, when the symptom started, what you have already tried, and what a finished repair would look like. Keep it to one page. The same page serves whether you do the work yourself or hire it out, so the choice is made on the same facts instead of on a hunch.
What goes on the page
Describe what you saw or heard, not what you think caused it. A rattle that starts when the unit reaches full speed is a symptom. A failing bearing is a guess, and a guess is better tested than assumed. Note how often the symptom happens, whether anyone else saw it, and the make, model, and age printed on the unit or on the purchase paper. If anything was changed, cleaned, or opened before, write that down too, because it affects both the repair and any contract question that follows.
Draw the boundary before you choose
Write down the work you are willing to do yourself and the work you will not do, with a reason for each line. Common lines are work that needs a gas, electrical, or refrigerant license, work on a sealed part, and work that needs a second person to lift safely. Licensing rules differ by state and by trade, so check the rule for your location rather than assume it. Then check your line against the contract. If the contract says who may open the unit or who may do the repair, that clause decides whether your own work could affect the cover, so read it before you touch anything.
Ask three questions of the contract
Put these to the seller in writing, and keep the answers with the contract.
Who may perform a repair under this contract, and does that change if I do part of the work myself?
What record must I keep, and who must receive it, if someone other than the provider does the work?
How do I report the symptom, and to which address or phone line?
Lay the two paths side by side
Make two columns, one for doing it yourself and one for hiring it out. Fill each row with a fact. Useful rows include the time you will spend, the trips needed to get parts, the disposal of the old part, who stands behind the workmanship, what happens if the fix does not hold, and who you call next. A blank cell is the question to ask before you decide. A column full of feelings is not a comparison.
What this guide does not do
This is a general explainer. It does not say what any state requires for any trade, and it does not say what any contract requires. This guide has not read every state's rules, so do not take any one point here as the rule for your location. Check the state page for your location on this site, and check the contract in your hand.