So back to the mechanic today, my temperature guage is still taking forever go up on the morning and never goes up beyong 1/4 when on the highway. He called me and said everything looks good, no error codes this and that the thermostat is working perfectly. I'm not sure how he knows this but I still refuse to believe this is a faulty temp sending unit. What is the best way to troubleshoot this? How can I convince him to change the thermostat again?