One room that never gets warm or cool is more often a duct problem than a furnace problem.
A single room that runs consistently hot or cold while the rest of the house is fine points at the ductwork feeding that room, not the furnace or air conditioner supplying the whole system. The usual suspects are a branch duct that was undersized or routed with too many turns for its length, a balancing damper that's closed or was never opened correctly at installation, and a leak on that one run losing air before it arrives.
All three get checked before anyone suggests replacing equipment over a single bad room, because a bigger furnace or a new air conditioner cannot fix a duct that's leaking, undersized or blocked between the equipment and the register. If every room in the house runs equally off rather than just one, that's a different question and more likely does point at the equipment itself — which is outside what we handle here.
Describe the problem and we'll tell you whether it's cleaning, sealing, repair, or not a duct problem at all.
Get Matched With a Duct ContractorA short description is enough — we'll follow up to sort out what it actually needs.