For the most part, unless you or a loved one do HVAC, you are at the mercy of your selected HVAC "professional.” At least some AC companies have proved to be unprofessional by focusing their business on sales over technical accuracy. There's a big gap between truth and lie, within that lies* the grey area of making something seem bigger or smaller than it really is. Just like a captain at the helm of a ship, they can steer you in the direction of replacing something that may not be in your best interests... but theirs. It will take a HVAC Sherlock Holmes to uncover the intricacies of the diagnostic accuracy, to the actual value and importance of making the repair.