what's the temperature gauge reading? If it also reads low you could have a blown head gasket.
Otherwise it could be heater core, but I doubt it. More likely it's the baffle that mixes hot with cold air - it's probably jammed a bit so it won't close off the cold air side. Do you park it outside? Sometimes rodents will build nests in cars and the nesting material gets stuck in places in the vents. That could be blocking it.
Or it could be the water valve isn't opening completely as well. Check that too
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