Buick Repair Questions? Ask a Mechanic for Answers ASAP
The thermostat on you engine are in a bad spot and very hard to get to. it's service with it's housing as one. the labor is about 2.5 hr. the part is about $65 plus coolant.
$400 is a bit high. $280-300 is max.
I would not recommend that you do this. it's really hard to get to. below is a picture of the location.