I foolishly allowed the dealer to do the oil cooler/filter housing repair on my '09 Mazda5 wagon recently. The service manager called back to say everything was done but the mechanic notice the temp had pegged while he ran the vehicle back to the carwash. They assumed the thermostat went bad. What a coincidence. I declined the $216 offer to replace the Tstat, drove the car home and did it myself. That got me wondering -if the tech allowed enough coolant to drain while he did the oil filter housing job, and didn't replace the coolant, could that have locked up the (non-defective) Tstat with an air bubble in the system? It seems like at the dealer one repair breeds another due to incompetence. PS, they didn't even top off the oil after the cooler/filter housing job.