I have not had a chance to work on mine yet. It has not been an issue since I have mostly been driving around town. However, I got on a hw yesterday and was able to experiment a bit. What I noticed is that at 42 or 43 mph, the heat goes off. Lower speeds, it comes right back on. So, some electronic sinal is either not being interpreted or sent properly. That is pure guessing.

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Julio Garcia
1987 300 SDL (mine)
2006 Jeep Liberty CRD (mine/kids)
2008 Hyundai Sonata (wife)
2000 Saturn SL AKA "Gocart" (everyone)