If you grab the low side outside under the hood and it is cold but not inside the car , you either have a heater mono valve problem or fresh air doors wide open.Usually fresh air doors wide open you will also have a/c blowing from defrost due to actuator problems. First pinch off the heater lines with a pair of needlenose vise grips.If no effect than put the car on recirculate mode and then remove glove box and see if the fresh air doors are closed.

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euro 287
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck