Nissan Repair: Maxima Thermostat Issue, coolant temperature sensor, freeway speeds

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
Wesley,

If the mechanic did not replace the thermostat have him do so.  If he did and your heater is not working properly and the heater is only warm then you have air trapped in the system.  If this is the case have the air bled from the system and you should be fine.  The thermostat is easy to replace as there are only two bolts and it is up top on the passenger side.  Also, the book shows the process and what to do to bleed the system of trapped air.  Get a Haynes Manual or get the factory one and you will see how easy it is to change a thermostat.