Ask your shop to measure the coolant temperature when the engine is fulled warmed, or at least after it has idled for 10-15 minutes after a cold start. A faulty engine cooling system thermostat is a pretty common problem by the reports we get here. The the thermostat is supposed to stay completely closed until the set point is reached. But over time the sealing surfaces begin to show wear and it begins to fail to seal properly, which causes lower than desired coolant operating temps. If the coolant is too cold there’s no heat to go around for the passenger compartment heaters.
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