Open Tstat usually throw code P0128 (engine temp under threshold), but not always. My old 3.0 had the open Tstat without code. I knew it's open since the temp needle never get to the 12 o'clock position, more like 11. Whenever I'm running on the hway, the needle will go down to 10 o'clock position. Heat will take a little while to come on.
Between Behr and Wahler, I would choose Wahler. Behr had parts (not all) made in China now, and we all know quality control over there is not the BEST in the world ( )
While you are replacing the serpentine belts, give the pulleys a spin. There should be resistance in them. If they spin freely, replace them.

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00 E39 DINAN S1 //M5 (82k miles)
06 E53 DINAN 4.8iS, born 2/18/2006 (126k miles) http://www.xoutpost.com/bmw-sav-foru...s-my-dslr.html
90 Straman Z32 TwinTurbo Convertible, (1 of 44 ever made) 5 speed, 444rwhp/451rwtq
01 360 Novitec Spider F1 (26k miles)