A do-it-yourself repair looks inexpensive at the start, because the part is often the only line you expect to pay for. The costs that surprise people come later: the tool you did not own, the part that did not match, the trip back to the shop, and the visit a claim now needs because the early attempt changed the unit.
Count the tools, not just the part
List every tool the job needs, including specialty items and meters. Mark which you own, which you would have to buy or borrow, and which the job cannot be done without. If you borrow, include the time to return it.
Count the trips
Estimate how many trips the job is likely to need: to buy the part, to return a wrong part, to confirm a diagnosis, and to call a trade back if the attempt does not hold. Each trip takes time and may take travel. The return visit is where a home fix most often becomes the expensive option.
Check the paper before you touch the unit
Many service contracts and warranties say who may perform work, and some say that unauthorised work may affect a claim. Read that clause before you start. Also check whether the manufacturer's instructions name a trade or a certification for the task.
Some work belongs to licensed trades
Gas, electrical, and refrigerant work are often restricted by state or local rules. Check the rules that apply where you live through the state's own regulator or licensing body, or ask the contract administrator which trades they accept. This page is general information, not legal advice.
Decide before you start
Write the decision down: what you will do yourself, what you will hire out, what would make you stop, and whom you will call if the job turns out larger than expected. If you cannot answer the tool and trip questions, the job is probably a booked visit.