Water temp#2

[ARPCAxPREZ at aol.com]

Thanks to all who replied about my recent colder water temperatures...looks like the thermostat is the violator.  It must be stuck open, now my questions:
Where is this located? ('97A4Tq)
Can I just tap the thing with a BFH and maybe the spring would close?  (fingersX)
Is it a tough job to replace
Who's gotta good price, if I haveta buy one.
Do you think this stuck thermostat triggered my check engine light, cause it's on now.
Thanks
Brad Blumfeldt 
Pittsburgh,PA