When an AC unit or roof is functionally working but clearly near the end of its life (say, 15+ years old with a few years left), flippers seem to handle it in two completely different ways.

Some operators replace them automatically on every single project. Their logic is that retail buyers and their inspectors panic the second they see an older serial number, so dropping the $6k to $9k upfront kills objections before they start, protects the appraisal, and prevents deals from falling out of escrow.

Other rehabbers leave working mechanicals alone. They'd rather keep that cash in reserve, price the home competitively, and just offer a home warranty or credit at the closing table if the buyer actually asks for it during option periods.