Originally Posted by Efthreeoh
So it's probably too late to give this advice since you got ripped off by the dealer by now, but for anyone else in this situation, here is how the t-stat and water pump fail.
The t-stat mechanically fails open, as per the cooling system information in the Bentley service manual. If the t-stat fails and throws a code it will illuminate the SES light because the t-stat is considered part of the emissions system since it regulates cold-start exhaust emissions via regulation of engine temperature. Ask the dealer what code (by number) it threw and then google the code and see what the code is for. If the SES light was not on, I doubt the t-stat is bad. The heating element in the t-stat goes bad and throws the code. If the dealer actually did replace the t-stat then it will be brown plastic instead of aluminum (but I'm not sure if by 2008 they changed the t-stat to plastic bodied from the factory). I let my t-stat go (throwing the SES every once in a while) for almost 10,000 miles before I replaced it; it will not overheat the engine.
As far as the water pump is concerned, it throws a shadow code that is not detected unless the car is scanned. Research water pump in the maintenance section and you'll find out all the info you need about when and how it fails. There is no predictability as to when the pumps fail. I would not replace a pump just because the t-stat is bad. The t-stat comes off first because it is bolted to the pump. It makes sense to replace the t-stat if the pump fails because the t-stat is relatively inexpensive ($100) as compared to the pump and adds no labor to the cost of a water pump replacement and has to come off to get the pump out.
Absolutely correct. Thanks for sharing!