Nissan Repair: Maxima Thermostat Issue, coolant temperature sensor, coolant level

Question
I have a 2000 Nissan Maxima GXE 5 speed with about 101,000 miles.  Recently the check engine light went on and the mechanic diagnosed it as code P1126, which for 2000 & 2001 models is a Thermostat Function issue.  The mechanic cleared the code and installed a new coolant temperature sensor.  I noticed today that after driving the car at freeway speeds for over 15 minutes, the temperature gauge was still at the cold line.  Also, the heater blows out warm instead of hot air.  The only time the temperature gauge raised a small amount was when I was at a stoplight and then it dropped again while driving.  The check engine light has not come on.  Based on other questions that have been answered here, I am thinking the thermostat should have been replaced also.  What do you think?  Is this something that a non-mechanical person could attempt, or should I bring it back to the mechanic?  THANKS!

Answer
first, i would check your coolant level to make sure that is level.  sometimes when its empty, it will read C. (personal experience...crack in block, couldn't detect it).  i would make sure the sensor was the correct one and was put in properly because the coolant temperature sensor was my first guess when i started reading this question.  your thermostat could also be the problem.  you can change this yourself, its not the hardest but you want to make sure you have some sort of pan to catch the coolant because this job could get messy.  make sure the thermostat comes with a gasket too.  good luck and e-mail me your findings.  xxp_sebby@yahoo.com