Sounds to me like the foam tube to the temperature sensor has disintegrated. Thus, the sensor is seeing warmer underdash temps. You could test by turning the dial to max heat until it clicks. That is supposed to override to sensor.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
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