Johnjay7777

So there is a bit of background i have to give you first. My 98 altima with 136k just got a tune up and the idler pully replace and the intake valve washers too.... It was spitting a code for a misfire in #4 cylinder... while my mechanic was replacing the pulley he replaced the serpentine belt too. i drove it home and didnt take notice in the fact that it was ideling high, as it has before the tune up. The next day i drove to work and while at a stop light i noticed that my temp gague was sitting on cold... i went to work and when i left later that day it didnt move from cold... not mind you it was 70 degrees... what should i do? might it be just the thermostat? the belt isnt tight? what do you think... anything is helpful.

thanks