Most likely a defroster duct temp sensor is no longer in the hole it's suppose to be. That'll do just what you're describing w/o setting any fault codes as it's electrically OK (plugged in). If your car has ever had an AC Evap replaced, that'd be a likely reason why it's now out if so.
The sensor is normally held in place by a plastic tie-strap. The right side is the easiest one to see by removing the glove box. Look towards the upper left of the glove box at the drefoster duct and you'll see it. The left side is in the same location, other side but is harder to see with the knee bolster in place from underneith.