Hey there I have a 2003 Limited 3.8L automatic with about 193,000 miles on it. Just bought it.

Is there a duct door that can break and keep air from flowing through the heater core into the cab? I have airflow but it's not hot.

Heater blows ambient air temp when it's warmed up after 15 minutes and at most becomes mildy warm after 2 hours of operating. Has rear heat too, and it never becomes mildly warm...stays ambient temp. Turning front or rear off at different times doesn't improve anything.

All the vent/duct settings work correctly...air blows from them strongly when selected. Air conditioner works fine in front and back. After two hours both heater core hoses on the firewall seem as hot as the upper and lower radiator hoses, but that could be from the hot motor bay air under the closed hood.

Is there a duct/air control or anything that can cause this? Like a door that keeps air from flowing through the heater core? From my searches online, and the shop manual, I don't see a part that can fail other than the heater core itself being clogged or the thermostat stuck open, that would degrade hot air in the cabin.

Anyone found different? I want to be sure before I disconnect hoses and rig flush access points from the hose junctions under the passenger side of the cab.

Just want to verify it's the heater core before further work actions with mild acid (white vinegar) and water pressure.

Thanks!